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Articles Particle physics 1 Standard Model 6 Supersymmetry 19 Elementary particle 26 Boson 32 Higgs boson 35 Higgs mechanism 43 Large Hadron Collider 52 Peter Higgs 65

Appendix 69 List of particles 69 References Article Sources and Contributors 76 Image Sources, Licenses and Contributors 79 Article Licenses License 80 Particle physics 1 Particle physics

Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics. Most of the interest in this area is in fundamental fields, each of which cannot be described as a bound state of other fields. The current set of fundamental fields and their dynamics are summarized in a theory called the Standard Model, therefore particle physics is largely the study of the Standard Model's particle content and its possible extensions. Collision of 2 beams of gold atoms recorded by RHIC

Subatomic particles

Modern particle physics research is focused on subatomic particles, including atomic constituents such as electrons, protons, and neutrons (protons and neutrons are composite particles called baryons, made of quarks), particles produced by radioactive and scattering processes, such as photons, neutrinos, and muons, as well as a wide range of exotic particles. To be specific, the term particle is a misnomer from classical physics because the dynamics of particle physics are governed by quantum mechanics. As such, they exhibit wave-particle duality, displaying particle-like behavior under certain experimental conditions and wave-like behavior in others. In more technical terms, they are described by quantum state vectors in a Hilbert space, which is also treated in quantum field theory. Following the convention of particle physicists, elementary The Standard Model of Physics. particles refer to objects such as electrons and photons as it is well known that these types of particles display wave-like properties as well.

All particles and their interactions observed to date can be described almost entirely by a quantum field theory called the Standard Model. The Standard Model has 17 species of elementary particles: 12 fermions or 24 if distinguishing antiparticles, 4 vector bosons (5 with antiparticles), and 1 scalar boson. These elementary particles can combine to Particle physics 2

form composite particles, accounting for the hundreds of other species of particles discovered since the 1960s. The Standard Model has been found to agree with almost all the experimental tests conducted to date. However, most particle physicists believe that it is an incomplete description of nature, and that a more fundamental theory awaits discovery (See Theory of Everything). In recent years, measurements of neutrino mass have provided the first experimental deviations from the Standard Model. Particle physics has impacted the philosophy of science greatly. Some particle physicists adhere to reductionism, a point of view that has been criticized and defended by philosophers and scientists.[1] [2] [3] [4] Other physicists may defend the philosophy of holism, which has commonly been viewed to be reductionism's opposite.[5]

History The idea that all matter is composed of elementary particles dates to at least the 6th century BC. The philosophical doctrine of atomism and the nature of elementary particles were studied by ancient Greek philosophers such as Leucippus, Democritus and Epicurus; ancient Indian philosophers such as Kanada, Dignāga and Dharmakirti; medieval scientists such as Alhazen, Avicenna, and Algazel; and early modern European physicists such as Pierre Gassendi, Robert Boyle, and Isaac Newton. The particle theory of light was also proposed by Alhazen, Avicenna, Gassendi, and Newton. These early ideas were founded in abstract, philosophical reasoning rather than experimentation and empirical observation. In the 19th century, John Dalton, through his work on stoichiometry, concluded that each element of nature was composed of a single, unique type of particle. Dalton and his contemporaries believed these were the fundamental particles of nature and thus named them atoms, after the Greek word atomos, meaning "indivisible". However, near the end of the century, physicists discovered that atoms are not, in fact, the fundamental particles of nature, but conglomerates of even smaller particles. The early 20th-century explorations of nuclear physics and quantum physics culminated in proofs of nuclear fission in 1939 by Lise Meitner (based on experiments by Otto Hahn), and nuclear fusion by Hans Bethe in the same year. These discoveries gave rise to an active industry of generating one atom from another, even rendering possible (although not profitable) the transmutation of lead into gold (alchemy). They also led to the development of nuclear weapons. Throughout the 1950s and 1960s, a bewildering variety of particles were found in scattering experiments. This was referred to as the "particle zoo". This term was deprecated after the formulation of the Standard Model during the 1970s in which the large number of particles was explained as combinations of a (relatively) small number of fundamental particles.

The Standard Model The very current state of the classification of elementary particles is the Standard Model. It describes the strong, weak, and electromagnetic fundamental interactions, using mediating gauge bosons. The species of gauge bosons are the gluons, W−, W+ and Z bosons, and the photons. The model also contains 24 fundamental particles, which are the constituents of matter. Finally, it predicts the existence of a type of boson known as the Higgs boson, which is yet to be discovered. Particle physics 3

Experimental laboratories In particle physics, the major international laboratories are: • Brookhaven National Laboratory (Long Island, United States). Its main facility is the Relativistic Heavy Ion Collider (RHIC), which collides heavy ions such as gold ions and polarized protons. It is the world's first heavy ion collider, and the world's only polarized proton collider. • Budker Institute of Nuclear Physics (Novosibirsk, Russia). Its main projects are now the electron-positron colliders VEPP-2000 [6], operated since 2006, and VEPP-4 [7], started experiments in 1994. Earlier facilities include the first electron-electron beam-beam collider VEP-1, which conducted experiments from 1964 to 1968; the electron-positron colliders VEPP-2, operated from 1965 to 1974; and its successor VEPP-2M [8], performed experiments in 1974-2000. • CERN, (Franco-Swiss border, near Geneva). Its main project is now the Large Hadron Collider (LHC), which had its first beam circulation on 10 September 2008, and is now the world's most energetic collider of protons. It will also be the most energetic collider of heavy ions when it begins colliding lead ions in 2010. Earlier facilities include the Large Electron–Positron Collider (LEP), which was stopped in 2001 and then dismantled to give way for LHC; and the Super Proton Synchrotron, which is being reused as a pre-accelerator for LHC. • DESY (Hamburg, Germany). Its main facility is the Hadron Elektron Ring Anlage (HERA), which collides electrons and positrons with protons. • Fermilab, (Batavia, United States). Its main facility is the Tevatron, which collides protons and antiprotons and was the highest-energy particle collider in the world until the Large Hadron Collider surpassed it on 29 November 2009. • KEK, (Tsukuba, Japan). It is the home of a number of experiments such as the K2K experiment, a neutrino oscillation experiment and Belle, an experiment measuring the CP violation of B mesons. Many other particle accelerators exist. The techniques required to do modern experimental particle physics are quite varied and complex, constituting a sub-specialty nearly completely distinct from the theoretical side of the field.

Theory Theoretical particle physics attempts to develop the models, theoretical framework, and mathematical tools to understand current experiments and make predictions for future experiments. See also theoretical physics. There are several major interrelated efforts in theoretical particle physics today. One important branch attempts to better understand the Standard Model and its tests. By extracting the parameters of the Standard Model from experiments with less uncertainty, this work probes the limits of the Standard Model and therefore expands our understanding of nature's building blocks. These efforts are made challenging by the difficulty of calculating quantities in quantum chromodynamics. Some theorists working in this area refer to themselves as phenomenologists and may use the tools of quantum field theory and effective field theory. Others make use of lattice field theory and call themselves lattice theorists. Another major effort is in model building where model builders develop ideas for what physics may lie beyond the Standard Model (at higher energies or smaller distances). This work is often motivated by the hierarchy problem and is constrained by existing experimental data. It may involve work on supersymmetry, alternatives to the Higgs mechanism, extra spatial dimensions (such as the Randall-Sundrum models), Preon theory, combinations of these, or other ideas. A third major effort in theoretical particle physics is string theory. String theorists attempt to construct a unified description of quantum mechanics and general relativity by building a theory based on small strings, and branes rather than particles. If the theory is successful, it may be considered a "Theory of Everything". Particle physics 4

There are also other areas of work in theoretical particle physics ranging from particle cosmology to loop quantum gravity. This division of efforts in particle physics is reflected in the names of categories on the arXiv, a preprint archive [9]: hep-th (theory), hep-ph (phenomenology), hep-ex (experiments), hep-lat (lattice gauge theory).

The future The overarching goal, which is pursued in several distinct ways, is to find and understand what physics may lie beyond the standard model. There are several powerful experimental reasons to expect new physics, including dark matter and neutrino mass. There are also theoretical hints that this new physics should be found at accessible energy scales. Furthermore, there may be unexpected and unpredicted surprises that will give us the most opportunity to learn about nature. Much of the efforts to find this new physics are focused on new collider experiments. The Large Hadron Collider (LHC) was completed in 2008 to help continue the search for the Higgs boson, supersymmetric particles, and other new physics. An intermediate goal is the construction of the International Linear Collider (ILC), which will complement the LHC by allowing more precise measurements of the properties of newly found particles. In August 2004, a decision for the technology of the ILC was taken but the site has still to be agreed upon. In addition, there are important non-collider experiments that also attempt to find and understand physics beyond the Standard Model. One important non-collider effort is the determination of the neutrino masses, since these masses may arise from neutrinos mixing with very heavy particles. In addition, cosmological observations provide many useful constraints on the dark matter, although it may be impossible to determine the exact nature of the dark matter without the colliders. Finally, lower bounds on the very long lifetime of the proton put constraints on Grand Unified Theories at energy scales much higher than collider experiments will be able to probe any time soon.

Notes

[1] "Review of particle physics" (http:/ / pdg. lbl. gov/ ). .

[2] "Particle Physics News and Resources" (http:/ / www. interactions. org/ ). .

[3] "CERN Courier - International Journal of High-Energy Physics" (http:/ / cerncourier. com). .

[4] "Particle physics in 60 seconds" (http:/ / www. symmetrymagazine. org/ cms/ ?pid=1000345). .

[5] "Quantum Holism" (http:/ / www. bu. edu/ wcp/ Papers/ Scie/ ScieTsek. htm). .

[6] http:/ / vepp2k. inp. nsk. su/

[7] http:/ / v4. inp. nsk. su/ index. en. html

[8] http:/ / www. inp. nsk. su/ activity/ old/ vepp2m/ index. ru. shtml

[9] http:/ / www. arxiv. org

References

Further reading

General readers • Frank Close (2004) Particle Physics: A Very Short Introduction. Oxford University Press. ISBN 0-19-280434-0. • ------, Michael Marten, and Christine Sutton (2002) The Particle Odyssey: A Journey to the Heart of the Matter. Oxford Univ. Press. ISBN 0-19-850486-1. • Ford, Kenneth W. (2005) The Quantum World. Harvard Univ. Press. • Oerter, Robert (2006) The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics. Plume. • Schumm, Bruce A. (2004) Deep Down Things: The Breathtaking Beauty of Particle Physics. John Hopkins Univ. Press. ISBN 0-8018-7971-X. Particle physics 5

• Riazuddin, PhD. "An Overview of Particle Physics and Cosmology" (http:/ / indico. ncp. edu. pk/ indico/ getFile.

py/ access?sessionId=0& resId=0& materialId=0& confId=10). NCP Journal of Physics (Dr. Professor Riazuddin, High Energy Theory Group, and senior scientist at the National Center for Nuclear Physics) 1 (1): 50.

Gentle texts • Frank Close (2006) The New Cosmic Onion. Taylor & Francis. ISBN 1-58488-798-2. • Lincoln Wolfenstein & Joao P. Silva (2010) Exploring Fundamental Particles . Taylor & Francis. ISBN 978-143-983-612-5 . • Coughlan, G. D., J. E. Dodd, and B. M. Gripaios (2006) The Ideas of Particle Physics: An Introduction for Scientists, 3rd ed. Cambridge Univ. Press. An undergraduate text for those not majoring in physics.

Harder A survey article: • Robinson, Matthew B., Gerald Cleaver, and J. R. Dittmann (2008) "A Simple Introduction to Particle Physics" -

Part 1, 135pp. (http:/ / arxiv. org/ abs/ 0810. 3328v1) and Part 2, nnnpp. (http:/ / arxiv. org/ abs/ 0908. 1395v1) Baylor University Dept. of Physics. Texts: • Griffiths, David J. (1987). Introduction to Elementary Particles. Wiley, John & Sons, Inc. ISBN 0-471-60386-4. • Kane, Gordon L. (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0-201-11749-5. • Perkins, Donald H. (1999). Introduction to High Energy Physics. Cambridge University Press. ISBN 0-521-62196-8. • Povh, Bogdan (1995). Particles and Nuclei: An Introduction to the Physical Concepts. Springer-Verlag. ISBN 0-387-59439-6. • Boyarkin, Oleg (2011). Advanced Particle Physics Two-Volume Set. CRC Press. ISBN 978-143-980-412-4.

External links

• The Particle Adventure (http:/ / particleadventure. org/ ) - educational project sponsored by the Particle Data Group of the Lawrence Berkeley National Laboratory (LBNL)

• symmetry magazine (http:/ / www. symmetrymagazine. org)

• Particle physics – it matters (http:/ / www. iop. org/ publications/ iop/ 2009/ page_38211. html) - the Institute of Physics

• Nobes, Matthew (2002) "Introduction to the Standard Model of Particle Physics" on Kuro5hin: Part 1, (http:/ /

www. kuro5hin. org/ story/ 2002/ 5/ 1/ 3712/ 31700) Part 2, (http:/ / www. kuro5hin. org/ story/ 2002/ 5/ 14/

19363/ 8142) Part 3a, (http:/ / www. kuro5hin. org/ story/ 2002/ 7/ 15/ 173318/ 784) Part 3b. (http:/ / www.

kuro5hin. org/ story/ 2002/ 8/ 21/ 195035/ 576)

• CERN (http:/ / public. web. cern. ch/ public/ ) - European Organization for Nuclear Research

• Fermilab (http:/ / www. fnal. gov/ ) Standard Model 6 Standard Model

The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon experimental confirmation of the existence of quarks. Since then, discoveries of the bottom quark (1977), the top quark (1995) and the tau neutrino (2000) have given credence to the Standard Model. Because of its success in explaining a wide variety of experimental results, the Standard Model is sometimes regarded as a theory of almost everything.

Still, the Standard Model falls short of being a complete theory of fundamental interactions because it does not incorporate

the physics of dark energy nor of the full The Standard Model of elementary particles, with the gauge bosons in the theory of gravitation as described by general rightmost column relativity. The theory does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology. It also does not correctly account for neutrino oscillations (and their non-zero masses). Although the Standard Model is theoretically self-consistent, it has several apparently unnatural properties giving rise to puzzles like the strong CP problem and the hierarchy problem.

Nevertheless, the Standard Model is important to theoretical and experimental particle physicists alike. For theorists, the Standard Model is a paradigmatic example of a quantum field theory, which exhibits a wide range of physics including spontaneous symmetry breaking, anomalies, non-perturbative behavior, etc. It is used as a basis for building more exotic models which incorporate hypothetical particles, extra dimensions and elaborate symmetries (such as supersymmetry) in an attempt to explain experimental results at variance with the Standard Model such as the existence of dark matter and neutrino oscillations. In turn, the experimenters have incorporated the standard model into simulators to help search for new physics beyond the Standard Model from relatively uninteresting background. Recently, the standard model has found applications in fields besides particle physics, such as astrophysics, cosmology, and nuclear physics. Standard Model 7

Historical background The first step towards the Standard Model was Sheldon Glashow's discovery in 1960 of a way to combine the electromagnetic and weak interactions.[1] In 1967 Steven Weinberg[2] and Abdus Salam[3] incorporated the Higgs mechanism[4] [5] [6] into Glashow's electroweak theory, giving it its modern form. The Higgs mechanism is believed to give rise to the masses of all the elementary particles in the Standard Model. This includes the masses of the W and Z bosons, and the masses of the fermions, i.e. the quarks and leptons. After the neutral weak currents caused by Z boson exchange were discovered at CERN in 1973,[7] [8] [9] [10] the electroweak theory became widely accepted and Glashow, Salam, and Weinberg shared the 1979 Nobel Prize in Physics for discovering it. The W and Z bosons were discovered experimentally in 1981, and their masses were found to be as the Standard Model predicted. The theory of the strong interaction, to which many contributed, acquired its modern form around 1973–74, when experiments confirmed that the hadrons were composed of fractionally charged quarks.

Overview At present, matter and energy are best understood in terms of the kinematics and interactions of elementary particles. To date, physics has reduced the laws governing the behavior and interaction of all known forms of matter and energy to a small set of fundamental laws and theories. A major goal of physics is to find the "common ground" that would unite all of these theories into one integrated theory of everything, of which all the other known laws would be special cases, and from which the behavior of all matter and energy could be derived (at least in principle).[11] The Standard Model groups two major extant theories—quantum electroweak and quantum chromodynamics—into an internally consistent theory that describes the interactions between all known particles in terms of quantum field theory. For a technical description of the fields and their interactions, see Standard Model (mathematical formulation).

Particle content

Fermions

Organization of Fermions

Charge First generation Second generation Third generation

Quarks +2⁄ Up u Charm c Top t 3

−1⁄ Down d Strange s Bottom b 3

Leptons −1 Electron e− Muon μ− Tau τ−

0 Electron neutrino ν Muon neutrino ν Tau neutrino ν e μ τ

The Standard Model includes 12 elementary particles of spin-1⁄ known as fermions. According to the spin-statistics 2 theorem, fermions respect the Pauli exclusion principle. Each fermion has a corresponding antiparticle. The fermions of the Standard Model are classified according to how they interact (or equivalently, by what charges they carry). There are six quarks (up, down, charm, strange, top, bottom), and six leptons (electron, electron neutrino, muon, muon neutrino, tau, tau neutrino). Pairs from each classification are grouped together to form a generation, with corresponding particles exhibiting similar physical behavior (see table). The defining property of the quarks is that they carry color charge, and hence, interact via the strong interaction. A phenomenon called color confinement results in quarks being perpetually (or at least since very soon after the start of Standard Model 8

the Big Bang) bound to one another, forming color-neutral composite particles (hadrons) containing either a quark and an antiquark (mesons) or three quarks (baryons). The familiar proton and the neutron are the two baryons having the smallest mass. Quarks also carry electric charge and weak isospin. Hence they interact with other fermions both electromagnetically and via the weak interaction. The remaining six fermions do not carry color charge and are called leptons. The three neutrinos do not carry electric charge either, so their motion is directly influenced only by the weak nuclear force, which makes them notoriously difficult to detect. However, by virtue of carrying an electric charge, the electron, muon, and tau all interact electromagnetically. Each member of a generation has greater mass than the corresponding particles of lower generations. The first generation charged particles do not decay; hence all ordinary (baryonic) matter is made of such particles. Specifically, all atoms consist of electrons orbiting atomic nuclei ultimately constituted of up and down quarks. Second and third generations charged particles, on the other hand, decay with very short half lives, and are observed only in very high-energy environments. Neutrinos of all generations also do not decay, and pervade the universe, but rarely interact with baryonic matter.

Gauge bosons

In the Standard Model, gauge bosons are defined as force carriers that mediate the strong, weak, and electromagnetic fundamental interactions. Interactions in physics are the ways that particles influence other particles. At a macroscopic level, electromagnetism allows particles to interact with one another via electric and magnetic fields, and gravitation allows particles with mass to attract one another in accordance with Einstein's theory of general relativity. The standard model explains such Summary of interactions between particles described by the Standard Model. forces as resulting from matter particles exchanging other particles, known as force mediating particles (strictly speaking, this is only so if interpreting literally what is actually an approximation method known as perturbation theory). When a force mediating particle is exchanged, at a macroscopic level the effect is equivalent to a force influencing both of them, and the particle is therefore said to have mediated (i.e., been the agent of) that force. The Feynman diagram calculations, which are a graphical representation of the perturbation theory approximation, invoke "force mediating particles", and when applied to analyze high-energy scattering experiments are in reasonable agreement with the data. However, perturbation theory (and with it the concept of a "force-mediating particle") fails in other situations. These include low-energy quantum chromodynamics, bound states, and solitons.

The gauge bosons of the Standard Model all have spin (as do matter particles). The value of the spin is 1, making them bosons. As a result, they do not follow the Pauli exclusion principle that constrains leptons: thus bosons (e.g. photons) do not have a theoretical limit on their spatial density (number per volume). The different types of gauge bosons are described below. Standard Model 9

• Photons mediate the electromagnetic force between electrically charged particles. The photon is massless and is well-described by the theory of quantum electrodynamics. • The W+, W−, and Z gauge bosons mediate the weak interactions between particles of different flavors (all quarks and leptons). They are massive, with the Z being more massive than the W±. The weak interactions involving the W± exclusively act on left-handed particles and right-handed antiparticles only. Furthermore, the W± carries an electric charge of +1 and −1 and couples to the electromagnetic interaction. The electrically neutral Z boson interacts with both left-handed particles and antiparticles. These three gauge bosons along with the photons are grouped together, as collectively mediating the electroweak interaction. • The eight gluons mediate the strong interactions between color charged particles (the quarks). Gluons are massless. The eightfold multiplicity of gluons is labeled by a combination of color and anticolor charge (e.g. red–antigreen).[12] Because the gluon has an effective color charge, they can also interact among themselves. The gluons and their interactions are described by the theory of quantum chromodynamics. The interactions between all the particles described by the Standard Model are summarized by the diagram at the top of this section.

Higgs boson The Higgs particle is a hypothetical massive scalar elementary particle theorized by Robert Brout, François Englert, Peter Higgs, Gerald Guralnik, C. R. Hagen, and Tom Kibble in 1964 (see 1964 PRL symmetry breaking papers) and is a key building block in the Standard Model.[13] [14] [15] [16] It has no intrinsic spin, and for that reason is classified as a boson (like the gauge bosons, which have integer spin). Because an exceptionally large amount of energy and beam luminosity are theoretically required to observe a Higgs boson in high energy colliders, it is the only fundamental particle predicted by the Standard Model that has yet to be observed. The Higgs boson plays a unique role in the Standard Model, by explaining why the other elementary particles, except the photon and gluon, are massive. In particular, the Higgs boson would explain why the photon has no mass, while the W and Z bosons are very heavy. Elementary particle masses, and the differences between electromagnetism (mediated by the photon) and the weak force (mediated by the W and Z bosons), are critical to many aspects of the structure of microscopic (and hence macroscopic) matter. In electroweak theory, the Higgs boson generates the masses of the leptons (electron, muon, and tau) and quarks. As yet, no experiment has conclusively detected the existence of the Higgs boson. It is hoped that the Large Hadron Collider at CERN will confirm the existence of this particle. As of August 2011, a significant portion of the possible masses for the Higgs have been excluded at 95% confidence level: CMS has excluded the mass ranges 145-216 GeV, 226-288 GeV and 310-400 GeV,[17] while the ATLAS experiment has excluded 146-232 GeV, 256-282 GeV and 296-466 GeV.[18] Note that these exclusions apply only to the Standard Model Higgs, and that more complex Higgs sectors which are possible in Beyond the Standard Model scenarios may be significantly more difficult to characterize. CERN director general Rolf Heuer has predicted that by the end of 2012 either the Standard Model Higgs boson will be observed, or excluded in all mass ranges, implying that the Standard Model is not the whole story.[19] Standard Model 10

Field content The standard model has the following fields:

Spin 1 1. A U(1) gauge field B with coupling g′ (weak U(1), or weak hypercharge) μν 2. An SU(2) gauge field W with coupling g (weak SU(2), or weak isospin) μν 3. An SU(3) gauge field G with coupling g (strong SU(3), or color charge) μν s

Spin 1⁄ 2 The spin 1⁄ particles are in representations of the gauge groups. For the U(1) group, we list the value of the weak 2 hypercharge instead. The left-handed fermionic fields are: 1. An SU(3) triplet, SU(2) doublet, with U(1) weak hypercharge 1⁄ (left-handed quarks) 3 2. An SU(3) triplet, SU(2) singlet, with U(1) weak hypercharge 2⁄ (left-handed down-type antiquark) 3 3. An SU(3) singlet, SU(2) doublet with U(1) weak hypercharge −1 (left-handed lepton) 4. An SU(3) triplet, SU(2) singlet, with U(1) weak hypercharge −4⁄ (left-handed up-type antiquark) 3 5. An SU(3) singlet, SU(2) singlet with U(1) weak hypercharge 2 (left-handed antilepton) By CPT symmetry, there is a set of right-handed fermions with the opposite quantum numbers. This describes one generation of leptons and quarks, and there are three generations, so there are three copies of each field. Note that there are twice as many left-handed lepton field components as left-handed antilepton field components in each generation, but an equal number of left-handed quark and antiquark fields.

Spin 0 1. An SU(2) doublet H with U(1) hyper-charge −1 (Higgs field) Note that |H|2, summed over the two SU(2) components, is invariant under both SU(2) and under U(1), and so it can appear as a renormalizable term in the Lagrangian, as can its square. This field acquires a vacuum expectation value, leaving a combination of the weak isospin, I , and weak hypercharge 3 unbroken. This is the electromagnetic gauge group, and the photon remains massless. The standard formula for the electric charge (which defines the normalization of the weak hypercharge, Y, which would otherwise be somewhat arbitrary) is:[20]

Lagrangian The Lagrangian for the spin 1 and spin 1⁄ fields is the most general renormalizable gauge field Lagrangian with no 2 fine tunings: • Spin 1:

where the traces are over the SU(2) and SU(3) indices hidden in W and G respectively. The two-index objects are the field strengths derived from W and G the vector fields. There are also two extra hidden parameters: the theta angles for SU(2) and SU(3). The spin-1⁄ particles can have no mass terms because there is no right/left helicity pair with the same SU(2) and 2 SU(3) representation and the same weak hypercharge. This means that if the gauge charges were conserved in the vacuum, none of the spin 1⁄ particles could ever swap helicity, and they would all be massless. 2 Standard Model 11

For a neutral fermion, for example a hypothetical right-handed lepton N (or Nα in relativistic two-spinor notation), with no SU(3), SU(2) representation and zero charge, it is possible to add the term:

This term gives the neutral fermion a Majorana mass. Since the generic value for M will be of order 1, such a particle would generically be unacceptably heavy. The interactions are completely determined by the theory – the leptons introduce no extra parameters.

Higgs mechanism The Lagrangian for the Higgs includes the most general renormalizable self interaction:

The parameter v2 has dimensions of mass squared, and it gives the location where the classical Lagrangian is at a minimum. In order for the Higgs mechanism to work, v2 must be a positive number. v has units of mass, and it is the only parameter in the standard model which is not dimensionless. It is also much smaller than the Planck scale; it is approximately equal to the Higgs mass, and sets the scale for the mass of everything else. This is the only real fine-tuning to a small nonzero value in the standard model, and it is called the Hierarchy problem. It is traditional to choose the SU(2) gauge so that the Higgs doublet in the vacuum has expectation value (v,0).

Masses and CKM matrix The rest of the interactions are the most general spin-0 spin-1⁄ Yukawa interactions, and there are many of these. 2 These constitute most of the free parameters in the model. The Yukawa couplings generate the masses and mixings once the Higgs gets its vacuum expectation value. The terms L*HR generate a mass term for each of the three generations of leptons. There are 9 of these terms, but by relabeling L and R, the matrix can be diagonalized. Since only the upper component of H is nonzero, the upper SU(2) component of L mixes with R to make the electron, the muon, and the tau, leaving over a lower massless component, the neutrino. Note: Neutrino oscillations show neutrinos have mass.[21] See also: Pontecorvo–Maki–Nakagawa–Sakata matrix. The terms QHU generate up masses, while QHD generate down masses. But since there is more than one right-handed singlet in each generation, it is not possible to diagonalize both with a good basis for the fields, and there is an extra CKM matrix.

Theoretical aspects

Construction of the Standard Model Lagrangian Standard Model 12

Parameters of the Standard Model

Symbol Description Renormalization Value scheme (point)

m Electron mass 511 keV e m Muon mass 105.7 MeV μ m Tau mass 1.78 GeV τ m Up quark mass μ = 2 GeV 1.9 MeV u MS m Down quark mass μ = 2 GeV 4.4 MeV d MS m Strange quark mass μ = 2 GeV 87 MeV s MS m Charm quark mass μ = m 1.32 GeV c MS c m Bottom quark mass μ = m 4.24 GeV b MS b m Top quark mass On-shell scheme 172.7 GeV t θ CKM 12-mixing angle 13.1° 12 θ CKM 23-mixing angle 2.4° 23 θ CKM 13-mixing angle 0.2° 13 δ CKM CP-violating Phase 0.995

g U(1) gauge coupling μ = m 0.357 1 MS Z g SU(2) gauge coupling μ = m 0.652 2 MS Z g SU(3) gauge coupling μ = m 1.221 3 MS Z θ QCD vacuum angle ~0 QCD μ Higgs quadratic coupling Unknown

λ Higgs self-coupling strength Unknown

Technically, quantum field theory provides the mathematical framework for the standard model, in which a Lagrangian controls the dynamics and kinematics of the theory. Each kind of particle is described in terms of a dynamical field that pervades space-time. The construction of the standard model proceeds following the modern method of constructing most field theories: by first postulating a set of symmetries of the system, and then by writing down the most general renormalizable Lagrangian from its particle (field) content that observes these symmetries. The global Poincaré symmetry is postulated for all relativistic quantum field theories. It consists of the familiar translational symmetry, rotational symmetry and the inertial reference frame invariance central to the theory of special relativity. The local SU(3)×SU(2)×U(1) gauge symmetry is an internal symmetry that essentially defines the standard model. Roughly, the three factors of the gauge symmetry give rise to the three fundamental interactions. The fields fall into different representations of the various symmetry groups of the Standard Model (see table). Upon writing the most general Lagrangian, one finds that the dynamics depend on 19 parameters, whose numerical values are established by experiment. The parameters are summarized in the table at right. Standard Model 13

Quantum chromodynamics sector The quantum chromodynamics (QCD) sector defines the interactions between quarks and gluons, with SU(3) symmetry, generated by Ta. Since leptons do not interact with gluons, they are not affected by this sector. The Dirac Lagrangian of the quarks coupled to the gluon fields is given by

is the SU(3) gauge field containing the gluons, are the Dirac matrices, D and U are the Dirac spinors associated with up- and down-type quarks, and g is the strong coupling constant. s Electroweak sector The electroweak sector is a Yang–Mills gauge theory with the symmetry group U(1)×SU(2) , L

where B is the U(1) gauge field; Y is the weak hypercharge—the generator of the U(1) group; is the μ W three-component SU(2) gauge field; are the Pauli matrices—infinitesimal generators of the SU(2) group. The subscript L indicates that they only act on left fermions; g′ and g are coupling constants.

Higgs sector In the Standard Model, the Higgs field is a complex spinor of the group SU(2) : L

where the indexes + and 0 indicate the electric charge (Q) of the components. The weak isospin (Y ) of both W components is 1. Before symmetry breaking, the Higgs Lagrangian is:

which can also be written as:

Additional symmetries of the Standard Model From the theoretical point of view, the Standard Model exhibits four additional global symmetries, not postulated at the outset of its construction, collectively denoted accidental symmetries, which are continuous U(1) global symmetries. The transformations leaving the Lagrangian invariant are:

The first transformation rule is shorthand meaning that all quark fields for all generations must be rotated by an identical phase simultaneously. The fields , and , are the 2nd (muon) and 3rd (tau) generation analogs of and fields. By Noether's theorem, each symmetry above has an associated conservation law: the conservation of baryon number, electron number, muon number, and tau number. Each quark is assigned a baryon number of 1/3, while each antiquark is assigned a baryon number of -1/3. Conservation of baryon number implies that the number of quarks Standard Model 14

minus the number of antiquarks is a constant. Within experimental limits, no violation of this conservation law has been found. Similarly, each electron and its associated neutrino is assigned an electron number of +1, while the anti-electron and the associated anti-neutrino carry a −1 electron number. Similarly, the muons and their neutrinos are assigned a muon number of +1 and the tau leptons are assigned a tau lepton number of +1. The Standard Model predicts that each of these three numbers should be conserved separately in a manner similar to the way baryon number is conserved. These numbers are collectively known as lepton family numbers (LF). Symmetry works differently for quarks than for leptons, mainly because the Standard Model predicts (incorrectly) that neutrinos are massless. However, in 2002 it was discovered that neutrinos have mass (now established to be not greater than 0.28 electron volts), and as neutrinos oscillate between flavors (muon neutrinos have been observed changing to tau neutrinos) the discovery of neutrino mass indicates that the conservation of lepton family number is violated.[22] In addition to the accidental (but exact) symmetries described above, the Standard Model exhibits several approximate symmetries. These are the "SU(2) custodial symmetry" and the "SU(2) or SU(3) quark flavor symmetry."

Symmetries of the Standard Model and Associated Conservation Laws

Symmetry Lie Group Symmetry Type Conservation Law

Poincaré Translations×SO(3,1) Global symmetry Energy, Momentum, Angular momentum

Gauge SU(3)×SU(2)×U(1) Local symmetry Color charge, Weak isospin, Electric charge, Weak hypercharge

Baryon phase U(1) Accidental Global symmetry Baryon number

Electron phase U(1) Accidental Global symmetry Electron number

Muon phase U(1) Accidental Global symmetry Muon number

Tau phase U(1) Accidental Global symmetry Tau number

Field content of the Standard Model

Field Spin Gauge group Baryon Electron (1st generation) Representation Number Number

Left-handed quark ( , , )

Left-handed up antiquark ( , , )

Left-handed down antiquark ( , , )

Left-handed lepton ( , , )

Left-handed antielectron ( , , )

Hypercharge gauge field ( , , )

Isospin gauge field ( , , )

Gluon field ( , , )

Higgs field ( , , ) Standard Model 15

List of standard model fermions This table is based in part on data gathered by the Particle Data Group.[23]

Left-handed fermions in the Standard Model

Generation 1

Fermion Symbol Electric Weak Weak Color Mass ** (left-handed) charge isospin hypercharge charge *

Electron 511 keV

Positron 511 keV

Electron neutrino < 0.28 eV ****

Electron antineutrino < 0.28 eV ****

Up quark ~ 3 MeV ***

Up antiquark ~ 3 MeV ***

Down quark ~ 6 MeV ***

Down antiquark ~ 6 MeV ***

Generation 2

Fermion Symbol Electric Weak Weak Color Mass ** (left-handed) charge isospin hypercharge charge *

Muon 106 MeV

Antimuon 106 MeV

Muon neutrino < 0.28 eV ****

Muon antineutrino < 0.28 eV ****

Charm quark ~ 1.337 GeV

Charm antiquark ~ 1.3 GeV

Strange quark ~ 100 MeV

Strange antiquark ~ 100 MeV

Generation 3

Fermion Symbol Electric Weak Weak Color Mass ** (left-handed) charge isospin hypercharge charge *

Tau 1.78 GeV

Antitau 1.78 GeV

Tau neutrino < 0.28 eV ****

Tau antineutrino < 0.28 eV ****

Top quark 171 GeV

Top antiquark 171 GeV

Bottom quark ~ 4.2 GeV

Bottom antiquark ~ 4.2 GeV Standard Model 16

Notes: • * These are not ordinary abelian charges, which can be added together, but are labels of group representations of Lie groups. • ** Mass is really a coupling between a left-handed fermion and a right-handed fermion. For example, the mass of an electron is really a coupling between a left-handed electron and a right-handed electron, which is the antiparticle of a left-handed positron. Also neutrinos show large mixings in their mass coupling, so it's not accurate to talk about neutrino masses in the flavor basis or to suggest a left-handed electron antineutrino. • *** The masses of baryons and hadrons and various cross-sections are the experimentally measured quantities. Since quarks can't be isolated because of QCD confinement, the quantity here is supposed to be the mass of the quark at the renormalization scale of the QCD scale. • **** The Standard Model assumes that neutrinos are massless. However, several contemporary experiments prove that neutrinos oscillate [24] between their flavour states, which could not happen if all were massless. It is straightforward to extend the model to fit these data but there are many possibilities, so the mass eigenstates are still open. See Neutrino#Mass.

Tests and predictions

The Standard Model (SM) predicted the existence of the W and Z bosons, gluon, and the top and charm quarks before these particles were observed. Their predicted properties were

experimentally confirmed with good Log plot of masses in the Standard Model. precision. To give an idea of the success of the SM, the following table compares the measured masses of the W and Z bosons with the masses predicted by the SM:

Quantity Measured (GeV) SM prediction (GeV)

Mass of W boson 80.398 ± 0.025 80.390 ± 0.018

Mass of Z boson 91.1876 ± 0.0021 91.1874 ± 0.0021

The SM also makes several predictions about the decay of Z bosons, which have been experimentally confirmed by the Large Electron-Positron Collider at CERN.

Challenges There is some experimental evidence consistent with neutrinos having mass, which the Standard Model does not allow.[25] To accommodate such findings, the Standard Model can be modified by adding a non-renormalizable interaction of lepton fields with the square of the Higgs field. This is natural in certain grand unified theories, and if new physics appears at about 1016 GeV, the neutrino masses are of the right order of magnitude. Currently, there is one elementary particle predicted by the Standard Model that has yet to be observed: the Higgs boson. A major reason for building the Large Hadron Collider is that the high energies of which it is capable are expected to make the Higgs observable. However, as of January 2011, there is only indirect empirical evidence for the existence of the Higgs boson, so that its discovery cannot be claimed. Moreover, some theoretical concerns have been raised positing that elementary scalar Higgs particles cannot exist (see Quantum triviality). A fair amount of theoretical and experimental research has attempted to extend the Standard Model into a Unified Field Theory or a Theory of everything, a complete theory explaining all physical phenomena including constants. Inadequacies of the Standard Model that motivate such research include: • It does not attempt to explain gravitation, and unlike for the strong and electroweak interactions of the Standard Model, there is no known way of describing general relativity, the canonical theory of gravitation, consistently in terms of quantum field theory. The reason for this is among other things that quantum field theories of gravity Standard Model 17

generally break down before reaching the Planck scale. As a consequence, we have no reliable theory for the very early universe; • It is considered by experts to be ad-hoc and inelegant, requiring 19 numerical constants whose values are unrelated and arbitrary. Although the Standard Model, as it now stands, can explain why neutrinos have masses, the specifics of neutrino mass are still unclear. It is believed that explaining neutrino mass will require an additional 7 or 8 constants, which are also arbitrary parameters; • The Higgs mechanism gives rise to the hierarchy problem if any new physics (such as quantum gravity) is present at high energy scales. In order for the weak scale to be much smaller than the Planck scale, severe fine tuning of Standard Model parameters is required; • It should be modified so as to be consistent with the emerging "standard model of cosmology." In particular, the Standard Model cannot explain the observed amount of cold dark matter (CDM) and gives contributions to dark energy which are far too large. It is also difficult to accommodate the observed predominance of matter over antimatter (matter/antimatter asymmetry). The isotropy and homogeneity of the visible universe over large distances seems to require a mechanism like cosmic inflation, which would also constitute an extension of the Standard Model. Currently no proposed Theory of everything has been conclusively verified.

Notes and references

Notes [1] S.L. Glashow (1961). "Partial-symmetries of weak interactions". Nuclear Physics 22: 579–588. Bibcode 1961NucPh..22..579G. doi:10.1016/0029-5582(61)90469-2. [2] S. Weinberg (1967). "A Model of Leptons". Physical Review Letters 19: 1264–1266. Bibcode 1967PhRvL..19.1264W. doi:10.1103/PhysRevLett.19.1264. [3] A. Salam (1968). N. Svartholm. ed. Elementary Particle Physics: Relativistic Groups and Analyticity. Eighth Nobel Symposium. Stockholm: Almquvist and Wiksell. pp. 367. [4] F. Englert, R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13: 321–323. Bibcode 1964PhRvL..13..321E. doi:10.1103/PhysRevLett.13.321. [5] P.W. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13: 508–509. Bibcode 1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508. [6] G.S. Guralnik, C.R. Hagen, T.W.B. Kibble (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13: 585–587. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585. [7] F.J. Hasert et al. (1973). "Search for elastic muon-neutrino electron scattering". Physics Letters B 46: 121. Bibcode 1973PhLB...46..121H. doi:10.1016/0370-2693(73)90494-2. [8] F.J. Hasert et al. (1973). "Observation of neutrino-like interactions without muon or electron in the gargamelle neutrino experiment". Physics Letters B 46: 138. Bibcode 1973PhLB...46..138H. doi:10.1016/0370-2693(73)90499-1. [9] F.J. Hasert et al. (1974). "Observation of neutrino-like interactions without muon or electron in the Gargamelle neutrino experiment". Nuclear Physics B 73: 1. Bibcode 1974NuPhB..73....1H. doi:10.1016/0550-3213(74)90038-8.

[10] D. Haidt (4 October 2004). "The discovery of the weak neutral currents" (http:/ / cerncourier. com/ cws/ article/ cern/ 29168). CERN Courier. . Retrieved 2008-05-08. [11] "Details can be worked out if the situation is simple enough for us to make an approximation, which is almost never, but often we can understand more or less what is happening." from The Feynman Lectures on Physics, Vol 1. pp. 2–7 [12] Technically, there are nine such color–anticolor combinations. However, there is one color-symmetric combination that can be constructed out of a linear superposition of the nine combinations, reducing the count to eight. [13] F. Englert, R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13: 321–323. Bibcode 1964PhRvL..13..321E. doi:10.1103/PhysRevLett.13.321. [14] P.W. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13: 508–509. Bibcode 1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508. [15] G.S. Guralnik, C.R. Hagen, T.W.B. Kibble (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13: 585–587. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585. [16] G.S. Guralnik (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles". International Journal of Modern Physics A 24: 2601–2627. arXiv:0907.3466. Bibcode 2009IJMPA..24.2601G. doi:10.1142/S0217751X09045431. Standard Model 18

[17] (http:/ / cms. web. cern. ch/ cms/ News/ 2011/ LP11/ )

[18] (http:/ / atlas. web. cern. ch/ Atlas/ GROUPS/ PHYSICS/ CONFNOTES/ ATLAS-CONF-2011-135/ )

[19] (http:/ / www. zdnet. co. uk/ news/ emerging-tech/ 2011/ 07/ 25/ cern-higgs-boson-answer-to-come-by-end-of-2012-40093510/ ) [20] The normalization Q = I + Y is sometimes used instead. 3

[21] http:/ / operaweb. lngs. infn. it/ spip. php?rubrique14 31May2010 Press Release.

[22] "Neutrino 'ghost particle' sized up by astronomers" (http:/ / www. bbc. co. uk/ news/ 10364160). BBC News. 22 June 2010. .

[23] W.-M. Yao et al. (Particle Data Group) (2006). "Review of Particle Physics: Quarks" (http:/ / pdg. lbl. gov/ 2006/ tables/ qxxx. pdf). Journal of Physics G 33: 1. arXiv:astro-ph/0601168. Bibcode 2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001. .

[24] W.-M. Yao et al. (Particle Data Group) (2006). "Review of Particle Physics: Neutrino mass, mixing, and flavor change" (http:/ / pdg. lbl.

gov/ 2007/ reviews/ numixrpp. pdf). Journal of Physics G 33: 1. arXiv:astro-ph/0601168. Bibcode 2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001. .

[25] CERN Press Release (http:/ / press. web. cern. ch/ press/ PressReleases/ Releases2010/ PR08. 10E. html)

References

Further reading • R. Oerter (2006). The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics. Plume. • B.A. Schumm (2004). Deep Down Things: The Breathtaking Beauty of Particle Physics. Johns Hopkins University Press. ISBN 0-8018-7971-X. Introductory textbooks • I. Aitchison, A. Hey (2003). Gauge Theories in Particle Physics: A Practical Introduction.. Institute of Physics. ISBN 9780585445502. • W. Greiner, B. Müller (2000). Gauge Theory of Weak Interactions. Springer. ISBN 3-540-67672-4. • G.D. Coughlan, J.E. Dodd, B.M. Gripaios (2006). The Ideas of Particle Physics: An Introduction for Scientists. Cambridge University Press. • D.J. Griffiths (1987). Introduction to Elementary Particles. John Wiley & Sons. ISBN 0-471-60386-4. • G.L. Kane (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0-201-11749-5. Advanced textbooks • T.P. Cheng, L.F. Li (2006). Gauge theory of elementary particle physics. Oxford University Press. ISBN 0-19-851961-3. Highlights the gauge theory aspects of the Standard Model. • J.F. Donoghue, E. Golowich, B.R. Holstein (1994). Dynamics of the Standard Model. Cambridge University Press. ISBN 978-0521476522. Highlights dynamical and phenomenological aspects of the Standard Model. • L. O'Raifeartaigh (1988). Group structure of gauge theories. Cambridge University Press. ISBN 0-521-34785-8. Highlights group-theoretical aspects of the Standard Model. Journal articles • E.S. Abers, B.W. Lee (1973). "Gauge theories". Physics Reports 9: 1–141. Bibcode 1973PhR.....9....1A. doi:10.1016/0370-1573(73)90027-6. • Y. Hayato et al. (1999). "Search for Proton Decay through p → νK+ in a Large Water Cherenkov Detector". Physical Review Letters 83: 1529. arXiv:hep-ex/9904020. Bibcode 1999PhRvL..83.1529H. doi:10.1103/PhysRevLett.83.1529. • S.F. Novaes (2000). "Standard Model: An Introduction". arXiv:hep-ph/0001283 [hep-ph]. • D.P. Roy (1999). "Basic Constituents of Matter and their Interactions — A Progress Report.". arXiv:hep-ph/9912523 [hep-ph]. • F. Wilczek (2004). "The Universe Is A Strange Place". arXiv:astro-ph/0401347 [astro-ph]. Standard Model 19

External links

• " Standard Model may be found incomplete, (http:/ / www. newscientist. com/ news/ news. jsp?id=ns9999404)" New Scientist.

• " Observation of the Top Quark (http:/ / www-cdf. fnal. gov/ top_status/ top. html)" at Fermilab.

• " The Standard Model Lagrangian. (http:/ / cosmicvariance. com/ 2006/ 11/ 23/ thanksgiving)" After electroweak symmetry breaking, with no explicit Higgs boson.

• " Standard Model Lagrangian (http:/ / nuclear. ucdavis. edu/ ~tgutierr/ files/ stmL1. html)" with explicit Higgs terms. PDF, PostScript, and LaTeX versions.

• " The particle adventure. (http:/ / particleadventure. org/ )" Web tutorial.

• Nobes, Matthew (2002) "Introduction to the Standard Model of Particle Physics" on Kuro5hin: Part 1, (http:/ /

www. kuro5hin. org/ story/ 2002/ 5/ 1/ 3712/ 31700) Part 2, (http:/ / www. kuro5hin. org/ story/ 2002/ 5/ 14/

19363/ 8142) Part 3a, (http:/ / www. kuro5hin. org/ story/ 2002/ 7/ 15/ 173318/ 784) Part 3b. (http:/ / www.

kuro5hin. org/ story/ 2002/ 8/ 21/ 195035/ 576)

Supersymmetry

In particle physics, supersymmetry (often abbreviated SUSY) is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartners. In a theory with unbroken supersymmetry, for every type of boson there exists a corresponding type of fermion with the same mass and internal quantum numbers, and vice-versa. There is no direct evidence for the existence of supersymmetry.[1] It is motivated by possible solutions to several theoretical problems. Since the superpartners of the Standard Model particles have not been observed, supersymmetry, if it exists, must be a broken symmetry, allowing the superparticles to be heavier than the corresponding Standard Model particles. If supersymmetry exists close to the TeV energy scale, it allows for a solution of the hierarchy problem of the Standard Model, i.e., the fact that the Higgs boson mass is subject to quantum corrections which — barring extremely fine-tuned cancellations among independent contributions — would make it so large as to undermine the internal consistency of the theory. In supersymmetric theories, on the other hand, the contributions to the quantum corrections coming from Standard Model particles are naturally canceled by the contributions of the corresponding superpartners. Other attractive features of TeV-scale supersymmetry are the fact that it allows for the high-energy unification of the weak interactions, the strong interactions and electromagnetism, and the fact that it provides a candidate for Dark Matter and a natural mechanism for electroweak symmetry breaking. Therefore, scenarios where supersymmetric partners appear with masses not much greater than 1 TeV are considered the most well-motivated by theorists[2] . These scenarios would imply that experimental traces of the superpartners should begin to emerge in high-energy collisions at the LHC relatively soon. As of September 2011, no meaningful signs of the superpartners have been observed[3] [4] , which is beginning to significantly constrain the most popular incarnations of supersymmetry. However, the total parameter space of consistent supersymmetric extensions of the Standard Model is extremely diverse and can not be definitively ruled out at the LHC. Another theoretically appealing property of supersymmetry is that it offers the only "loophole" to the Coleman–Mandula theorem, which prohibits spacetime and internal symmetries from being combined in any nontrivial way, for quantum field theories like the Standard Model under very general assumptions. The Haag-Lopuszanski-Sohnius theorem demonstrates that supersymmetry is the only way spacetime and internal symmetries can be consistently combined.[5] In general, supersymmetric quantum field theory is often much easier to work with, as many more problems become exactly solvable. Supersymmetry is also a feature of most versions of string theory, though it may exist in nature Supersymmetry 20

even if string theory is incorrect. The Minimal Supersymmetric Standard Model is one of the best studied candidates for physics beyond the Standard Model. Theories of gravity that are also invariant under supersymmetry are known as supergravity theories.

History A supersymmetry relating mesons and baryons was first proposed, in the context of hadronic physics, by Hironari Miyazawa in 1966, but his work was ignored at the time.[6] [7] [8] [9] In the early 1970s, J. L. Gervais and B. Sakita (in 1971), Yu. A. Golfand and E.P. Likhtman (also in 1971), D.V. Volkov and V.P. Akulov (in 1972) and J. Wess and B. Zumino (in 1974) independently rediscovered supersymmetry, a radically new type of symmetry of spacetime and fundamental fields, which establishes a relationship between elementary particles of different quantum nature, bosons and fermions, and unifies spacetime and internal symmetries of the microscopic world. Supersymmetry first arose in 1971 in the context of an early version of string theory by Pierre Ramond, John H. Schwarz and Andre Neveu, but the mathematical structure of supersymmetry has subsequently been applied successfully to other areas of physics; firstly by Wess, Zumino, and Abdus Salam and their fellow researchers to particle physics, and later to a variety of fields, ranging from quantum mechanics to statistical physics. It remains a vital part of many proposed theories of physics. The first realistic supersymmetric version of the Standard Model was proposed in 1981 by Howard Georgi and Savas Dimopoulos and is called the Minimal Supersymmetric Standard Model or MSSM for short. It was proposed to solve the hierarchy problem and predicts superpartners with masses between 100 GeV and 1 TeV. As of 2009 there is no irrefutable experimental evidence that supersymmetry is a symmetry of nature. Since 2010, the Large Hadron Collider at CERN is producing the world's highest energy collisions and offers the best chance at discovering superparticles for the foreseeable future.

Applications

Extension of possible symmetry groups One reason that physicists explored supersymmetry is because it offers an extension to the more familiar symmetries of quantum field theory. These symmetries are grouped into the Poincaré group and internal symmetries and the Coleman–Mandula theorem showed that under certain assumptions, the symmetries of the S-matrix must be a direct product of the Poincaré group with a compact internal symmetry group or if there is no mass gap, the conformal group with a compact internal symmetry group. In 1971 Golfand and Likhtman were the first to show that the Poincaré algebra can be extended through introduction of four anticommuting spinor generators (in four dimensions), which later became known as supercharges. In 1975 the Haag-Lopuszanski-Sohnius theorem analyzed all possible superalgebras in the general form, including those with an extended number of the supergenerators and central charges. This extended super-Poincaré algebra paved the way for obtaining a very large and important class of supersymmetric field theories.

The supersymmetry algebra Traditional symmetries in physics are generated by objects that transform under the tensor representations of the Poincaré group and internal symmetries. Supersymmetries, on the other hand, are generated by objects that transform under the spinor representations. According to the spin-statistics theorem, bosonic fields commute while fermionic fields anticommute. Combining the two kinds of fields into a single algebra requires the introduction of a Z -grading 2 under which the bosons are the even elements and the fermions are the odd elements. Such an algebra is called a Lie superalgebra. The simplest supersymmetric extension of the Poincaré algebra is the Super-Poincaré algebra. Expressed in terms of two Weyl spinors, has the following anti-commutation relation: Supersymmetry 21

and all other anti-commutation relations between the Qs and commutation relations between the Qs and Ps vanish. In the above expression are the generators of translation and are the Pauli matrices. There are representations of a Lie superalgebra that are analogous to representations of a Lie algebra. Each Lie algebra has an associated Lie group and a Lie superalgebra can sometimes be extended into representations of a Lie supergroup.

The Supersymmetric Standard Model Incorporating supersymmetry into the Standard Model requires doubling the number of particles since there is no way that any of the particles in the Standard Model can be superpartners of each other. With the addition of new particles, there are many possible new interactions. The simplest possible supersymmetric model consistent with the Standard Model is the Minimal Supersymmetric Standard Model (MSSM) which can include the necessary additional new particles that are able to be superpartners of those in the Standard Model. One of the main motivations for SUSY comes from the quadratically divergent contributions to the Higgs mass squared. The quantum mechanical interactions of the Higgs boson causes a large renormalization of the Higgs mass and unless there is an accidental cancellation, the natural size of the Higgs mass is the highest scale possible. This problem is known as the hierarchy problem. Supersymmetry reduces the size of the quantum corrections by having automatic cancellations between fermionic and bosonic Higgs interactions. If supersymmetry is restored at the weak scale, then the Higgs mass is related to Cancellation of the Higgs boson quadratic mass renormalization between fermionic supersymmetry breaking which can be top quark loop and scalar stop squark tadpole Feynman diagrams in a induced from small non-perturbative effects supersymmetric extension of the Standard Model explaining the vastly different scales in the weak interactions and gravitational interactions.

In many supersymmetric Standard Models there is a heavy stable particle (such as neutralino) which could serve as a Weakly interacting massive particle (WIMP) dark matter candidate. The existence of a supersymmetric dark matter candidate is closely tied to R-parity. The standard paradigm for incorporating supersymmetry into a realistic theory is to have the underlying dynamics of the theory be supersymmetric, but the ground state of the theory does not respect the symmetry and supersymmetry is broken spontaneously. The supersymmetry break can not be done permanently by the particles of the MSSM as they currently appear. This means that there is a new sector of the theory that is responsible for the breaking. The only constraint on this new sector is that it must break supersymmetry permanently and must give superparticles TeV scale masses. There are many models that can do this and most of their details do not currently matter. In order to parameterize the relevant features of supersymmetry breaking, arbitrary soft SUSY breaking terms are added to the theory which temporarily break SUSY explicitly but could never arise from a complete theory of supersymmetry breaking. Supersymmetry 22

Gauge Coupling Unification One piece of evidence for supersymmetry existing is gauge coupling unification. The renormalization group evolution of the three gauge coupling constants of the Standard Model is somewhat sensitive to the present particle content of the theory. These coupling constants do not quite meet together at a common energy scale if we run the renormalization group using the Standard Model.[1] With the addition of minimal SUSY joint convergence of the coupling constants is projected at approximately 1016 GeV.[1]

Supersymmetric quantum mechanics Supersymmetric quantum mechanics adds the SUSY superalgebra to quantum mechanics as opposed to quantum field theory. Supersymmetric quantum mechanics often comes up when studying the dynamics of supersymmetric solitons and due to the simplified nature of having fields only functions of time (rather than space-time), a great deal of progress has been made in this subject and is now studied in its own right. SUSY quantum mechanics involves pairs of Hamiltonians which share a particular mathematical relationship, which are called partner Hamiltonians. (The potential energy terms which occur in the Hamiltonians are then called partner potentials.) An introductory theorem shows that for every eigenstate of one Hamiltonian, its partner Hamiltonian has a corresponding eigenstate with the same energy. This fact can be exploited to deduce many properties of the eigenstate spectrum. It is analogous to the original description of SUSY, which referred to bosons and fermions. We can imagine a "bosonic Hamiltonian", whose eigenstates are the various bosons of our theory. The SUSY partner of this Hamiltonian would be "fermionic", and its eigenstates would be the theory's fermions. Each boson would have a fermionic partner of equal energy. SUSY concepts have provided useful extensions to the WKB approximation. In addition, SUSY has been applied to non-quantum statistical mechanics through the Fokker-Planck equation.

Mathematics SUSY is also sometimes studied mathematically for its intrinsic properties. This is because it describes complex fields satisfying a property known as holomorphy, which allows holomorphic quantities to be exactly computed. This makes supersymmetric models useful toy models of more realistic theories. A prime example of this has been the demonstration of S-duality in four-dimensional gauge theories that interchanges particles and monopoles.

General supersymmetry Supersymmetry appears in many different contexts in theoretical physics that are closely related. It is possible to have multiple supersymmetries and also have supersymmetric extra dimensions.

Extended supersymmetry It is possible to have more than one kind of supersymmetry transformation. Theories with more than one supersymmetry transformation are known as extended supersymmetric theories. The more supersymmetry a theory has, the more constrained the field content and interactions are. Typically the number of copies of a supersymmetry is a power of 2, i.e. 1, 2, 4, 8. In four dimensions, a spinor has four degrees of freedom and thus the minimal number of supersymmetry generators is four in four dimensions and having eight copies of supersymmetry means that there are 32 supersymmetry generators. The maximal number of supersymmetry generators possible is 32. Theories with more than 32 supersymmetry generators automatically have massless fields with spin greater than 2. It is not known how to make massless fields with spin greater than two interact, so the maximal number of supersymmetry generators considered is 32. This corresponds to an N = 8 supersymmetry theory. Theories with 32 supersymmetries automatically have a graviton. Supersymmetry 23

In four dimensions there are the following theories, with the corresponding multiplets[10] (CPT adds a copy, whenever they are not invariant under such symmetry) • N = 1 Chiral multiplet: (0,1⁄ ) Vector multiplet: (1⁄ ,1) Gravitino multiplet: (1,3⁄ ) Graviton multiplet: (3⁄ ,2) 2 2 2 2 • N = 2 hypermultiplet: (-1⁄ ,02,1⁄ ) vector multiplet: (0,1⁄ 2,1) supergravity multiplet: (1,3⁄ 2,2) 2 2 2 2 • N = 4 Vector multiplet: (-1,-1⁄ 4,06,1⁄ 4,1) Supergravity multiplet: (0,1⁄ 4,16,3⁄ 4,2) 2 2 2 2 • N = 8 Supergravity multiplet: (-2,-3⁄ 8,-128,-1⁄ 56,070,1⁄ 56,128,3⁄ 8,2) 2 2 2 2

Supersymmetry in alternate numbers of dimensions It is possible to have supersymmetry in dimensions other than four. Because the properties of spinors change drastically between different dimensions, each dimension has its characteristic. In d dimensions, the size of spinors is roughly 2d/2 or 2(d − 1)/2. Since the maximum number of supersymmetries is 32, the greatest number of dimensions in which a supersymmetric theory can exist is eleven.

Supersymmetry as a quantum group Supersymmetry can be reinterpreted in the language of noncommutative geometry and quantum groups. In particular, it involves a mild form of noncommutativity, namely supercommutativity. See the main article for more details.

Supersymmetry in quantum gravity Supersymmetry is part of a larger enterprise of theoretical physics to unify everything we know about the physical world into a single fundamental framework of physical laws, known as the quest for a Theory of Everything (TOE). A significant part of this larger enterprise is the quest for a theory of quantum gravity, which would unify the classical theory of general relativity and the Standard Model, which explains the other three basic forces in physics (electromagnetism, the strong interaction, and the weak interaction), and provides a palette of fundamental particles upon which all four forces act. Two of the most active approaches to forming a theory of quantum gravity are string theory and loop quantum gravity (LQG), although in theory, supersymmetry could be a component of other theoretical approaches as well. For string theory to be consistent, supersymmetry appears to be required at some level (although it may be a strongly broken symmetry). In particle theory, supersymmetry is recognized as a way to stabilize the hierarchy between the unification scale and the electroweak scale (or the Higgs boson mass), and can also provide a natural dark matter candidate. String theory also requires extra spatial dimensions which have to be compactified as in Kaluza-Klein theory. Loop quantum gravity (LQG), in its current formulation, predicts no additional spatial dimensions, nor anything else about particle physics. These theories can be formulated in three spatial dimensions and one dimension of time, although in some LQG theories dimensionality is an emergent property of the theory, rather than a fundamental assumption of the theory. Also, LQG is a theory of quantum gravity which does not require supersymmetry. Lee Smolin, one of the originators of LQG, has proposed that a loop quantum gravity theory incorporating either supersymmetry or extra dimensions, or both, be called "loop quantum gravity II". If experimental evidence confirms supersymmetry in the form of supersymmetric particles such as the neutralino that is often believed to be the lightest superpartner, some people believe this would be a major boost to string theory. Supersymmetry 24

Since supersymmetry is a required component of string theory, any discovered supersymmetry would be consistent with string theory. If the Large Hadron Collider and other major particle physics experiments fail to detect supersymmetric partners or evidence of extra dimensions, many versions of string theory which had predicted certain low mass superpartners to existing particles may need to be significantly revised. The failure of experiments to discover either supersymmetric partners or extra spatial dimensions, as of 2009, has encouraged loop quantum gravity researchers.

Current Limits The tightest limits will of course come from direct production at colliders. Both the Large Electron–Positron Collider and Tevatron had set limits for specific models which have now been exceeded by the Large Hadron Collider. Searches are only applicable for a finite set of tested points because simulation using the Monte Carlo method must be made so that limits for that particular model can be calculated. This complicates matters because different experiments have looked at different sets of points. Some extrapolation between points can be made within particular models but it is difficult to set general limits even for the Minimal Supersymmetric Standard Model. The first mass limits for squarks and gluinos were made at CERN by the UA1 experiment and the UA2 experiment at the Super Proton Synchrotron. LEP later set very strong limits.[11] In 2006 these limits were extended by the D0 experiment[12] [13] The LHC has now extended these limits, by extending the search area, with no sign of supersymmmetry.[14] [15] [16] [17]

References [1] Gordon L. Kane, The Dawn of Physics Beyond the Standard Model, Scientific American, June 2003, page 60 and The frontiers of physics, special edition, Vol 15, #3, page 8 "Indirect evidence for supersymmetry comes from the extrapolation of interactions to high energies."

[2] (http:/ / profmattstrassler. com/ articles-and-posts/ lhcposts/ what-do-current-mid-august-2011-lhc-results-imply-about-supersymmetry/ )

[3] ATLAS SUSY search documents (https:/ / twiki. cern. ch/ twiki/ bin/ view/ AtlasPublic/ SupersymmetryPublicResults#Early_2011_Data_5_CONF_Notes)

[4] CMS SUSY search documents (https:/ / twiki. cern. ch/ twiki/ bin/ view/ CMSPublic/ PhysicsResultsSUS)

[5] R. Haag, J. T. Lopuszanski and M. Sohnius, " All Possible Generators Of Supersymmetries Of The S Matrix (http:/ / www. sciencedirect.

com/ science/ article/ B6TVC-4718W97-YF/ 1/ bc160d55fb6a0faddac181fcff6871ce)", Nucl. Phys. B 88 (1975) 257 [6] H. Miyazawa (1966). "Baryon Number Changing Currents". Prog. Theor. Phys. 36 (6): 1266–1276. Bibcode 1966PThPh..36.1266M. doi:10.1143/PTP.36.1266. [7] H. Miyazawa (1968). "Spinor Currents and Symmetries of Baryons and Mesons". Phys. Rev. 170 (5): 1586–1590. Bibcode 1968PhRv..170.1586M. doi:10.1103/PhysRev.170.1586. [8] Michio Kaku, Quantum Field Theory, ISBN 0-19-509158-2, pg 663. [9] Peter Freund, Introduction to Supersymmetry, ISBN 0-521-35675-X, pages 26-27, 138. [10] Polchinski,J. String theory. Vol. 2: Superstring theory and beyond, Appendix B [11] LEPSUSYWG, ALEPH, DELPHI, L3 and OPAL experiments, Charginos, large m0 LEPSUSYWG/01-03.1 [12] The D0-Collaboration, Search for associated production of charginos and neutralinos in the trilepton final state using 2.3 $fb^-1$ of data, arXiv:/0901.0646 [hep-ex] [13] The D0 Collaboration, V. Abazov, et al., Search for Squarks and Gluinos in events with jets and missing transverse energy using 2.1 $fb^-1$ of $p\bar{p}$ collision data at $\sqrt{s}$ = 1.96 TeV , arXiv:0712.3805v2 [hep-ex]

[14] Implications of Initial LHC Searches for Supersymmetry (http:/ / www. math. columbia. edu/ ~woit/ wordpress/ ?p=3479)

[15] Fine-tuning implications for complementary dark matter and LHC SUSY searches (http:/ / arxiv. org/ abs/ 1101. 4664)

[16] What LHC tells about SUSY (http:/ / resonaances. blogspot. com/ 2011/ 02/ what-lhc-tells-about-susy. html)

[17] Early SUSY searches at the LHC (http:/ / www. hep. ph. ic. ac. uk/ susytalks/ iop-susytapper. pdf) Supersymmetry 25

Further reading

General readers • Feynman, R.P. & Weinberg, S. (1987) Elementary Particles and the Laws of Physics: The 1986 Dirac Memorial Lectures. Cambridge Univ. Press. • Ford, Kenneth W. (2005) The Quantum World. Harvard Univ. Press. • Brian Greene (1999). The Elegant Universe. W.W.Norton & Company. ISBN 0-393-05858-1. • John Gribbin (2000) Q is for Quantum - An Encyclopedia of Particle Physics. Simon & Schuster. ISBN 0-684-85578-X. • Oerter, Robert (2006) The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics. Plume. • Schumm, Bruce A. (2004) Deep Down Things: The Breathtaking Beauty of Particle Physics. John Hopkins Univ. Press. ISBN 0-8018-7971-X. • Martinus Veltman (2003). Facts and Mysteries in Elementary Particle Physics. World Scientific. ISBN 981-238-149-X. • Frank Close (2004). Particle Physics: A Very Short Introduction. Oxford: Oxford University Press. ISBN 0-19-280434-0.

Textbooks • Bettini, Alessandro (2008) Introduction to Elementary Particle Physics. Cambridge Univ. Press. ISBN 9780521880213 • Coughlan, G. D., J. E. Dodd, and B. M. Gripaios (2006) The Ideas of Particle Physics: An Introduction for Scientists, 3rd ed. Cambridge Univ. Press. An undergraduate text for those not majoring in physics. • Griffiths, David J. (1987) Introduction to Elementary Particles. John Wiley & Sons. ISBN 0-471-60386-4. • Kane, Gordon L. (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0-201-11749-5. • Perkins, Donald H. (2000) Introduction to High Energy Physics, 4th ed. Cambridge Univ. Press. Elementary particle 32

External links The most important address about the current experimental and theoretical knowledge about elementary particle physics is the Particle Data Group, where different international institutions collect all experimental data and give short reviews over the contemporary theoretical understanding.

• Particle Data Group (http:/ / pdg. lbl. gov/ ) other pages are:

• Greene, Brian, " Elementary particles (http:/ / www. pbs. org/ wgbh/ nova/ elegant/ part-flash. html)", The Elegant Universe, NOVA (PBS)

• particleadventure.org (http:/ / particleadventure. org), a well-made introduction also for non physicists

• CERNCourier: Season of Higgs and melodrama (http:/ / www. cerncourier. com/ main/ article/ 41/ 2/ 17)

• Pentaquark information page (http:/ / plato. phy. ohiou. edu/ ~hicks/ thplus. htm)

• Interactions.org (http:/ / www. interactions. org), particle physics news

• Symmetry Magazine (http:/ / www. symmetrymagazine. org), a joint Fermilab/SLAC publication

• "Sized Matter: perception of the extreme unseen" (http:/ / www-personal. umich. edu/ ~janhande/ sizedmatter/

sizedmatter_images. htm), Michigan University project for artistic visualisation of subatomic particles

• Elementary Particles made thinkable (http:/ / www. thingsmadethinkable. com/ item/ elementary_particles. php), an interactive visualisation allowing physical properties to be compared

Boson

For other meanings, see Boson (disambiguation). In particle physics, bosons are subatomic particles that obey Bose–Einstein statistics. Several bosons can occupy the same quantum state. The word boson derives from the name of Satyendra Nath Bose.[1] Bosons contrast with fermions, which obey Fermi–Dirac statistics. Two or more fermions cannot occupy the same quantum state. Since bosons with the same energy can occupy the same place in space, bosons are often force carrier particles. In contrast, fermions are usually associated with matter (although in quantum physics the distinction between the two concepts is not clear cut).

Bosons may be either elementary, like photons, or composite, like mesons. Some composite bosons do not satisfy the criteria for Bose-Einstein statistics and are not truly bosons (e.g. helium-4 atoms); a more The Standard Model of elementary particles, with the gauge bosons in the last accurate term for such composite particles column would be "bosonic-composites". Boson 33

All observed bosons have integer spin, as opposed to fermions, which have half-integer spin. This is in accordance with the spin-statistics theorem which states that in any reasonable relativistic quantum field theory, particles with integer spin are bosons, while particles with half-integer spin are fermions. While most bosons are composite particles, in the Standard Model, there are six bosons which are elementary: • the four gauge bosons (γ · g · W± · Z) • the Higgs boson (H0) • the Graviton (G) Unlike the gauge bosons, the Higgs boson and Graviton have not yet been observed experimentally.[2] Composite bosons are important in superfluidity and other applications of Bose–Einstein condensates.

Definition and basic properties By definition, bosons are particles which obey Bose–Einstein statistics: when one swaps two bosons, the wavefunction of the system is unchanged.[3] Fermions, on the other hand, obey Fermi–Dirac statistics and the Pauli exclusion principle: two fermions cannot occupy the same quantum state, resulting in a "rigidity" or "stiffness" of matter which includes fermions. Thus fermions are sometimes said to be the constituents of matter, while bosons are said to be the particles that transmit interactions (force carriers), or the constituents of radiation. The quantum fields of bosons are bosonic fields, obeying canonical commutation relations. The properties of lasers and masers, superfluid helium-4 and Bose–Einstein condensates are all consequences of statistics of bosons. Another result is that the spectrum of a photon gas in thermal equilibrium is a Planck spectrum, one example of which is black-body radiation; another is the thermal radiation of the opaque early Universe seen today as microwave background radiation. Interactions between elementary particles are called fundamental interactions. The fundamental interactions of virtual bosons with real particles result in all forces we know. All known elementary and composite particles are bosons or fermions, depending on their spin: particles with half-integer spin are fermions; particles with integer spin are bosons. In the framework of nonrelativistic quantum mechanics, this is a purely empirical observation. However, in relativistic quantum field theory, the spin-statistics theorem shows that half-integer spin particles cannot be bosons and integer spin particles cannot be fermions.[4] In large systems, the difference between bosonic and fermionic statistics is only apparent at large densities—when their wave functions overlap. At low densities, both types of statistics are well approximated by Maxwell-Boltzmann statistics, which is described by classical mechanics.

Elementary bosons All observed elementary particles are either fermions or bosons. The observed elementary bosons are all gauge bosons: photons, W and Z bosons and gluons. • Photons are the force carriers of the electromagnetic field. • W and Z bosons are the force carriers which mediate the weak force. • Gluons are the fundamental force carriers underlying the strong force. In addition, the standard model postulates the existence of Higgs bosons, which give other particles their mass via the Higgs mechanism. Finally, many approaches to quantum gravity postulate a force carrier for gravity, the graviton, which is a boson of spin 2. Boson 34

Composite bosons Composite particles (such as hadrons, nuclei, and atoms) can be bosons or fermions depending on their constituents. More precisely, because of the relation between spin and statistics, a particle containing an even number of fermions is a boson, since it has integer spin. Examples include the following: • Any meson, since mesons contain one quark and one antiquark • The nucleus of a carbon-12 atom, which contains 6 protons and 6 neutrons • The helium-4 atom, consisting of 2 protons, 2 neutrons and 2 electrons The number of bosons within a composite particle made up of simple particles bound with a potential has no effect on whether it is a boson or a fermion. Fermionic or bosonic behavior of a composite particle (or system) is only seen at large (compared to size of the system) distance. At proximity, where spatial structure begins to be important, a composite particle (or system) behaves according to its constituent makeup. For example, two atoms of helium-4 cannot share the same space if it is comparable in size to that of the inner structure of the helium atom itself (~10−10 m)—despite bosonic properties of the helium-4 atoms. Thus, liquid helium has finite density comparable to the density of ordinary liquid matter.

Notes

[1] "boson (dictionary entry)" (http:/ / www. merriam-webster. com/ dictionary/ boson). Merriam-Webster's Online Dictionary. . Retrieved 2010-03-21.

[2] Standard Model of Particle Physics (http:/ / www-sldnt. slac. stanford. edu/ alr/ standard_model. htm), SLAC Large Detector (SLD) group

(http:/ / www-sld. slac. stanford. edu/ sldwww/ sld. html), Stanford Linear Accelerator Center (http:/ / www. slac. stanford. edu). [3] Srednicki (2007), pages 28-29 [4] Sakurai (1994), page 362

References • Sakurai, J.J. (1994). Modern Quantum Mechanics (Revised Edition), pp 361–363. Addison-Wesley Publishing Company, ISBN 0-201-53929-2.

• Srednicki, Mark (2007). Quantum Field Theory (http:/ / www. physics. ucsb. edu/ ~mark/ qft. html), Cambridge University Press, ISBN 978-0521864497. Higgs boson 35 Higgs boson

Higgs boson

A simulated event, featuring the appearance of the Higgs boson Composition Elementary particle

Statistics Bosonic

Status Hypothetical

Theorized F. Englert, R. Brout, P. Higgs, G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble (1964)

Discovered Not yet (as of November 2011); searches ongoing at the LHC

Types 1, according to the Standard Model; 5 or more, according to supersymmetric models

[1] Mass 115–185 GeV/c2 (model-dependent upper bound )

Spin 0

The Higgs boson is a hypothetical massive elementary particle that is predicted to exist by the Standard Model (SM) of particle physics. Its existence is postulated as a means of resolving inconsistencies in the Standard Model. Experiments attempting to find the particle are currently being performed using the Large Hadron Collider (LHC) at CERN, and were performed at Fermilab's Tevatron until Tevatron's closure in late 2011. The Higgs boson is the only elementary particle predicted by the Standard Model that has not been observed in particle physics experiments. It is a necessary requirement of the so-called Higgs mechanism, the part of the SM which explains how most of the known elementary particles obtain their mass.[2] For example, the Higgs mechanism would explain why the W and Z bosons, which mediate weak interactions, are massive whereas the related photon, which mediates electromagnetism, is massless. The Higgs boson is expected to be in a class of particles known as scalar bosons. (Bosons are particles with integer spin, and scalar bosons have spin 0.) Theories that do not need the Higgs boson are described as Higgsless models. Some theories suggest that any mechanism capable of generating the masses of the elementary particles must be visible at energies below 1.4 TeV;[3] therefore, the LHC is expected to be able to provide experimental evidence of the existence or non-existence of the Higgs boson.[4] Experiments at Fermilab were also continuing previous attempts at detection, albeit hindered by the lower energy of the Tevatron accelerator. The Fermilab collider ceased operation 30th September 2011 although data analysis still continues. Higgs boson 36

Origin of the theory

The Higgs mechanism is a process by which vector bosons can get a mass. It was proposed in 1964 independently and almost simultaneously by three groups of physicists: François Englert and Robert Brout;[5] by Peter Higgs[6] (inspired by ideas of Philip Anderson[7] ); and by Gerald Guralnik, C. R. Hagen, and Tom Kibble.[8]

The three papers written on this discovery were each recognized as milestone papers during Physical Review Letters's 50th anniversary Five of the six 2010 APS J.J. Sakurai Prize [9] celebration. While each of these famous papers took similar winners. From left to right: Kibble, Guralnik, approaches, the contributions and differences between the 1964 PRL Hagen, Englert, and Brout. symmetry breaking papers are noteworthy. These six physicists were also awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics for this work.[10]

The 1964 PRL papers by Higgs and by Guralnik, Hagen, and Kibble (GHK) both displayed equations for the field that would eventually become known as the Higgs boson. In the paper by Higgs the boson is massive, and in a closing sentence Higgs writes that "an essential feature" of the theory "is the prediction of incomplete multiplets of scalar and vector bosons". In the model described in the GHK paper the boson is massless and decoupled from the massive states. In recent reviews of the topic, Guralnik states that in the GHK model the boson No. six: Peter Higgs 2009 is massless only in a lowest-order approximation, but it is not subject to any constraint and it acquires mass at higher orders. Additionally, he states that the GHK paper was the only one to show that there are no massless Nambu-Goldstone bosons in the model and to give a complete analysis of the general Higgs mechanism. [11] [12] Following the publication of the 1964 PRL papers, the properties of the model were further discussed by Guralnik in 1965 and by Higgs in 1966.[13] [14]

Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the electroweak symmetry breaking. The Higgs mechanism not only explains how the electroweak vector bosons get a mass, but predicts the ratio between the W boson and Z boson masses as well as their couplings with each other and with the Standard Model quarks and leptons. Many of these predictions have been verified by precise measurements performed at the LEP and the SLC colliders, thus confirming that the Higgs mechanism takes place in nature.[15] The Higgs boson's existence is not a strictly necessary consequence of the Higgs mechanism: the Higgs boson exists in some but not all theories which use the Higgs mechanism. For example, the Higgs boson exists in the Standard Model and the Minimal Supersymmetric Standard Model yet is not expected to exist in Higgsless models, such as Technicolor. A goal of the LHC and Tevatron experiments is to distinguish among these models and determine if the Higgs boson exists or not. Higgs boson 37

Theoretical overview

The Higgs boson particle is the quantum of the theoretical Higgs field. In empty space, the Higgs field has an amplitude different from zero; i.e. a non-zero vacuum expectation value. The existence of this non-zero vacuum expectation plays a fundamental role; it gives mass to every elementary particle that couples to the Higgs field, including the Higgs boson itself. The acquisition of a non-zero vacuum expectation value spontaneously A one-loop Feynman diagram of the first-order breaks electroweak gauge symmetry. This is the Higgs correction to the Higgs mass. The Higgs boson couples mechanism, which is the simplest process capable of giving mass strongly to the top quark so it might decay into top–anti-top quark pairs if it were heavy enough. to the gauge bosons while remaining compatible with gauge theories. This field is analogous to a pool of molasses that "sticks" to the otherwise massless fundamental particles that travel through the field, converting them into particles with mass that form (for example) the components of atoms.

In the Standard Model, the Higgs field consists of two neutral and two charged component fields. Both of the charged components and one of the neutral fields are Goldstone bosons, which act as the longitudinal third-polarization components of the massive W+, W–, and Z bosons. The quantum of the remaining neutral component corresponds to the massive Higgs boson. Since the Higgs field is a scalar field, the Higgs boson has no spin, hence no intrinsic angular momentum. The Higgs boson is also its own antiparticle and is CP-even. The Standard Model does not predict the mass of the Higgs boson. If that mass is between 115 and 180 GeV/c2, then the Standard Model can be valid at energy scales all the way up to the Planck scale (1016 TeV). Many theorists expect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model. The highest possible mass scale allowed for the Higgs boson (or some other electroweak symmetry breaking mechanism) is 1.4 TeV; beyond this point, the Standard Model becomes inconsistent without such a mechanism, because unitarity is violated in certain scattering processes. There are over a hundred theoretical Higgs-mass predictions.[16] Extensions to the Standard Model including supersymmetry (SUSY) predict the existence of families of Higgs bosons, rather than the one Higgs particle of the Standard Model. Among the SUSY models, in the Minimal Supersymmetric Standard Model (MSSM) the Higgs mechanism yields the smallest number of Higgs bosons: there are two Higgs doublets, leading to the existence of a quintet of scalar particles, two CP-even neutral Higgs bosons h and H, a CP-odd neutral Higgs boson A, and two charged Higgs particles H±. Many supersymmetric models predict that the lightest Higgs boson will have a mass only slightly above the current experimental limits, at around 120 GeV/c2 or less.

Experimental search

As of November 2011, the Higgs boson has yet to be confirmed experimentally,[17] despite large efforts invested in accelerator experiments at CERN and Fermilab. Prior to the year 2000, the data gathered at the LEP collider at CERN Status as of March 2011, to the indicated confidence intervals Higgs boson 38

allowed an experimental lower bound to be set for the mass of the Standard Model Higgs boson of 114.4 GeV/c2 at the 95% confidence level. The same experiment has produced a small number of events that could be interpreted as resulting from Higgs bosons with a mass just above this cut off — around 115 GeV—but the number of events was insufficient to draw definite conclusions.[18] The LEP was shut down in 2000 due to construction of its successor, the LHC, which is expected to be able to confirm or reject the existence of the Higgs boson. Full operational mode was delayed until mid-November 2009, because of a serious fault discovered with a number of magnets during the [19] [20] A Feynman diagram of one way the Higgs boson may calibration and startup phase. be produced at the LHC. Here, two gluons convert to two top/anti-top pairs, which then combine to make a At the Fermilab Tevatron, there are ongoing experiments neutral Higgs. searching for the Higgs boson. As of July 2010, combined data from CDF and DØ experiments at the Tevatron were sufficient to exclude the Higgs boson in the range 158 GeV/c2 - 175 GeV/c2 at the 95% confidence level.[21] [22] Preliminary results as of July 2011 have since extended the excluded region to the range 156 GeV/c2 - 177 GeV/c2 at the 90% confidence level.[23] Data collection and analysis in search of Higgs are intensifying since March 30, 2010 when the LHC began operating at 3.5 TeV.[24] Preliminary results from the ATLAS and CMS experiments at the LHC as of July 2011 exclude a Standard Model Higgs boson in the mass range 155 GeV/c2 - 190 GeV/c2[25] and 149 GeV/c2 - A Feynman diagram of another way the Higgs boson 206 GeV/c2,[26] respectively, at the 95% confidence level. may be produced at the LHC. Here, two quarks each emit a W or Z boson, which combine to make a neutral Higgs.

It may be possible to estimate the mass of the Higgs boson indirectly. In the Standard Model, the Higgs boson has a number of indirect effects; most notably, Higgs loops result in tiny corrections to masses of W and Z bosons. Precision measurements of electroweak parameters, such as the Fermi constant and masses of W/Z bosons, can be used to constrain the mass of the Higgs. As of 2006, measurements of electroweak observables allowed the exclusion of a Standard Model Higgs boson having a mass greater than 285 GeV/c2 at 95% CL, and estimated its mass to be 129 GeV/c2 (the central value corresponding to approximately 138 proton masses).[27] As of August 2009, the Standard Model Higgs boson is excluded by electroweak measurements above 186 GeV at the 95% confidence level. However, it should be noted that these indirect constraints make the assumption that the Standard Model is correct. It may still be possible to discover a Higgs boson above 186 GeV if it is accompanied by other particles between the Standard Model and GUT scales. In a 2009 preprint,[28] [29] it was suggested that the Higgs boson might not only interact with the above-mentioned particles of the Standard model of particle physics, but also with the mysterious weakly interacting massive particles (or WIMPS) that may form dark matter, and which play an important role in recent astrophysics. Various reports of potential evidence for the existence of the Higgs boson have appeared in recent years[30] [31] [32] but to date none have provided convincing evidence. In April 2011, there were suggestions in the media that evidence for the Higgs boson might have been discovered at the LHC in Geneva, Switzerland[33] but these had been debunked by mid May.[34] In regard to these rumors Jon Butterworth, a member of the High Energy Physics group Higgs boson 39

on the Atlas experiment, stated they were not a hoax, but were based on unofficial, unreviewed results.[35] The LHC detected possible signs of the particle, which were reported in July 2011, the ATLAS Note concluding: "In the low mass range (c 120−140 GeV) an excess of events with a significance of approximately 2.8 sigma above the background expectation is observed" and the BBC reporting that "interesting particle events at a mass of between 140 and 145 GeV" were found.[36] [37] These findings were repeated shortly thereafter by researchers at the Tevatron with a spokesman stating that: "There are some intriguing things going on around a mass of 140GeV."[36] However, on 22 August it was reported that the anomalous results had become insignificant on the inclusion of more data from ATLAS and CMS and that the non-existence of the particle had been confirmed by LHC collisions to 95% certainty between 145–466 GeV (except for a few small islands around 250 GeV).[38] A combined analysis of ATLAS and CMS data, published in November 2011, further narrowed the window for the allowed values of the Higgs boson mass to 114-141 GeV.[39]

Alternatives for electroweak symmetry breaking In the years since the Higgs boson was proposed, several alternatives to the Higgs mechanism have been proposed. All of these proposed mechanisms use strongly interacting dynamics to produce a vacuum expectation value that breaks electroweak symmetry. A partial list of these alternative mechanisms are: • Technicolor,[40] a class of models that attempts to mimic the dynamics of the strong force as a way of breaking electroweak symmetry. • Extra dimensional Higgsless models where the role of the Higgs field is played by the fifth component of the gauge field.[41] • Abbott-Farhi models of composite W and Z vector bosons.[42] • Top quark condensate theory in which a fundamental scalar Higgs field is replaced by a composite field composed of the top quark and its antiquark. • The braid model of Standard Model particles by Sundance Bilson-Thompson, compatible with loop quantum gravity and similar theories.[43] • In theory of superfluid vacuum masses of elementary particles can arise as a result of interaction with the physical vacuum, similarly to the gap generation mechanism in superconductors.[44] [45]

"The God particle" The Higgs boson is often referred to as "the God particle" by the media,[46] after the title of Leon Lederman's book, The God Particle: If the Universe Is the Answer, What Is the Question?[47] Lederman initially wanted to call it the "goddamn particle," but his editor would not let him.[48] While use of this term may have contributed to increased media interest in particle physics and the Large Hadron Collider,[47] many scientists dislike it, since it overstates the particle's importance, not least since its discovery would still leave unanswered questions about the unification of QCD, the electroweak interaction and gravity, and the ultimate origin of the universe.[46] A renaming competition conducted by the science correspondent for the British Guardian newspaper chose the name "the champagne bottle boson" as the best from among their submissions: "The bottom of a champagne bottle is in the shape of the Higgs potential and is often used as an illustration in physics lectures. So it's not an embarrassingly grandiose name, it is memorable, and [it] has some physics connection too."[49] Higgs boson 40

Notes [1] This upper bound for the Higgs boson mass is a prediction within the minimal Standard Model assuming that it remains a consistent theory up to the Planck scale. In extensions of the SM, this bound can be loosened or, in the case of supersymmetry theories, lowered. The lower bound which results from direct experimental exclusion by LEP is valid for most extensions of the SM, but can be circumvented in special cases.

(http:/ / pdg. lbl. gov/ 2010/ reviews/ rpp2010-rev-higgs-boson. pdf) [2] The masses of composite particles such as the proton and neutron would only be partly due to the Higgs mechanism, and are already understood as a consequence of the strong interaction. [3] Lee, Benjamin W.; Quigg, C.; Thacker, H. B. (1977). "Weak interactions at very high energies: The role of the Higgs-boson mass". Physical Review D 16 (5): 1519–1531. Bibcode 1977PhRvD..16.1519L. doi:10.1103/PhysRevD.16.1519.

[4] "Huge $10 billion collider resumes hunt for 'God particle' - CNN.com" (http:/ / www. cnn. com/ 2009/ TECH/ 11/ 11/ lhc. large. hadron.

collider. beam/ index. html). CNN. 2009-11-11. . Retrieved 2010-05-04. [5] Englert, François; Brout, Robert (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13 (9): 321–23. Bibcode 1964PhRvL..13..321E. doi:10.1103/PhysRevLett.13.321. [6] Higgs, Peter (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13 (16): 508–509. Bibcode 1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508. [7] Ph. Anderson: "Plasmons, gauge invariance and mass." In: Physical Review. 130, 1963, p. 439–442 [8] Guralnik, Gerald; Hagen, C. R.; Kibble, T. W. B. (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13 (20): 585–587. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585.

[9] Physical Review Letters - 50th Anniversary Milestone Papers (http:/ / prl. aps. org/ 50years/ milestones#1964). Physical Review Letters. .

[10] "American Physical Society - J. J. Sakurai Prize Winners" (http:/ / www. aps. org/ units/ dpf/ awards/ sakurai. cfm). . [11] G.S. Guralnik (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles". International Journal of Modern Physics A 24 (14): 2601–2627. arXiv:0907.3466. Bibcode 2009IJMPA..24.2601G. doi:10.1142/S0217751X09045431. [12] "Guralnik, G.S. The Beginnings of Spontaneous Symmetry Breaking in Particle Physics. Proceedings of the DPF-2011 Conference,

Providence, RI, August 8–13, 2011" (http:/ / arxiv. org/ abs/ 1110. 2253v1). Arxiv.org. 2011-10-11. . Retrieved 2011-12-07. [13] G.S. Guralnik (2011). "GAUGE INVARIANCE AND THE GOLDSTONE THEOREM - 1965 Feldafing talk". Modern Physics Letters A 26 (19): 1381–1392. arXiv:1107.4592v1. Bibcode 2011MPLA...26.1381G. doi:10.1142/S0217732311036188. [14] Higgs, Peter (1966). "Spontaneous Symmetry Breakdown without Massless Bosons". Physical Review 145 (4): 1156–1163. Bibcode 1966PhRv..145.1156H. doi:10.1103/PhysRev.145.1156.

[15] "LEP Electroweak Working Group" (http:/ / lepewwg. web. cern. ch/ LEPEWWG/ ). . [16] T. Schücker (2007). "Higgs-mass predictions". arXiv:0708.3344 [hep-ph].

[17] Scientists present first “bread-and-butter” results from LHC collisions (http:/ / www. symmetrymagazine. org/ breaking/ 2010/ 06/ 08/

scientists-present-first-bread-and-butter-results-from-lhc-collisions/ ) Symmetry Breaking, 8 June 2010

[18] W.-M. Yao et al. (2006). Searches for Higgs Bosons "Review of Particle Physics" (http:/ / pdg. lbl. gov/ 2006/ reviews/ higgs_s055. pdf). Journal of Physics G 33: 1. arXiv:astro-ph/0601168. Bibcode 2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001. Searches for Higgs Bosons.

[19] "CERN management confirms new LHC restart schedule" (http:/ / press. web. cern. ch/ press/ PressReleases/ Releases2009/ PR02. 09E. html). CERN Press Office. 9 February 2009. . Retrieved 2009-02-10.

[20] "CERN reports on progress towards LHC restart" (http:/ / press. web. cern. ch/ press/ PressReleases/ Releases2009/ PR09. 09E. html). CERN Press Office. 19 June 2009. . Retrieved 2009-07-21. [21] T. Aaltonen et al. (CDF and DØ Collaborations) (2010). "Combination of Tevatron searches for the standard model Higgs boson in the W+W− decay mode". Physical Review Letters 104 (6). arXiv:1001.4162. Bibcode 2010PhRvL.104f1802A. doi:10.1103/PhysRevLett.104.061802.

[22] "Fermilab experiments narrow allowed mass range for Higgs boson" (http:/ / www. fnal. gov/ pub/ presspass/ press_releases/

Higgs-mass-constraints-20100726-images. html). Fermilab. 26 July 2010. . Retrieved 2010-07-26. [23] The CDF & D0 Collaborations (27 July 2011). "Combined CDF and D0 Upper Limits on Standard Model Higgs Boson Production with up to 8.6 fb-1 of Data". arXiv:1107.5518 [hep-ex].

[24] "''CERN Bulletin'' Issue No. 18-20/2010 - Monday 3 May 2010" (http:/ / cdsweb. cern. ch/ journal/ CERNBulletin/ 2010/ 18/ News Articles/ 1262593?ln=en). Cdsweb.cern.ch. 2010-05-03. . Retrieved 2011-12-07.

[25] "Combined Standard Model Higgs Boson Searches in pp Collisions at root-s = 7 TeV with the ATLAS Experiment at the LHC" (https:/ /

atlas. web. cern. ch/ Atlas/ GROUPS/ PHYSICS/ CONFNOTES/ ATLAS-CONF-2011-112/ ). 24 July 2011. ATLAS-CONF-2011-112. .

[26] "Search for standard model Higgs boson in pp collisions at sqrt{s}=7 TeV" (http:/ / cdsweb. cern. ch/ record/ 1370076/ ). 23 July 2011. CMS-PAS-HIG-11-011. . 0 [27] " H Indirect Mass Limits from Electroweak Analysis. (http:/ / pdglive. lbl. gov/ popupblockdata. brl?nodein=S055HEW& inscript=Y& fsizein=1)" [28] C. B. Jackson; Geraldine Servant; Gabe Shaughnessy; Tim M. P. Tait; Marco Taoso (2009). "Higgs in Space!". arXiv:0912.0004 [hep-ph].

[29] Physics World, "Higgs could reveal itself in Dark-Matter collisions (http:/ / physicsworld. com/ cws/ article/ news/ 41218). British Institute of Physics. Retrieved 26 July 2011. Higgs boson 41

[30] Potential Higgs Boson discovery: " Higgs Boson: Glimpses of the God particle. (http:/ / www. newscientist. com/ channel/ fundamentals/

mg19325934. 600-higgs-boson-glimpses-of-the-god-particle. html)" New Scientist, 02 March 2007

[31] " 'God particle' may have been seen, (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 3546973. stm)" BBC news, 10 March 2004.

[32] US experiment hints at 'multiple God particles' (http:/ / news. bbc. co. uk/ 1/ hi/ science_and_environment/ 10313875. stm) BBC News 14 June 2010

[33] "Mass hysteria! Science world buzzing over rumours the elusive 'God Particle' has finally been found- dailymail.co.uk" (http:/ / www.

dailymail. co. uk/ sciencetech/ article-1379844/ Science-world-buzzing-rumours-elusive-God-particle-found. html). Mail Online. 2011-04-24. . Retrieved 2011-04-24.

[34] The Collider That Cried Higgs (http:/ / www. nature. com/ news/ 2011/ 110510/ full/ 473136a. html) Nature 473, 136-137 (2011)

[35] Butterworth, Jon (2011-04-24). "The Guardian, "Rumours of the Higgs at ATLAS"" (http:/ / www. guardian. co. uk/ science/

life-and-physics/ 2011/ apr/ 24/ 1?CMP=twt_fd). Guardian. . Retrieved 2011-12-07.

[36] Rincon, Paul (24 July 2011) "Higgs boson 'hints' also seen by US lab" (http:/ / www. bbc. co. uk/ news/ science-environment-14266358) BBC News. Retrieved 24 July 2011.

[37] "Combined Standard Model Higgs Boson Searches in pp Collisions at √s = 7 TeV with the ATLAS Experiment at the LHC" (https:/ / atlas.

web. cern. ch/ Atlas/ GROUPS/ PHYSICS/ CONFNOTES/ ATLAS-CONF-2011-112/ ATLAS-CONF-2011-112. pdf) ATLAS Note (24 July 2011) (pdf) The ATLAS Collaboration. Retrieved 26 July 2011.

[38] Ghosh, Pallab (2011-08-22). "BBC News - Higgs boson range narrows at European collider" (http:/ / www. bbc. co. uk/ news/ science-environment-14596367). Bbc.co.uk. . Retrieved 2011-11-07.

[39] Geoff Brumfiel (2011-11-18). "Higgs hunt enters endgame" (http:/ / www. nature. com/ news/ higgs-hunt-enters-endgame-1. 9399). Nature News. . Retrieved 2011-11-22. [40] S. Dimopoulos and Leonard Susskind (1979). "Mass Without Scalars". Nuclear Physics B 155: 237–252. Bibcode 1979NuPhB.155..237D. doi:10.1016/0550-3213(79)90364-X. [41] C. Csaki and C. Grojean and L. Pilo and J. Terning (2004). "Towards a realistic model of Higgsless electroweak symmetry breaking". Physical Review Letters 92 (10): 101802. arXiv:hep-ph/0308038. Bibcode 2004PhRvL..92j1802C. doi:10.1103/PhysRevLett.92.101802. PMID 15089195. [42] L. F. Abbott and E. Farhi (1981). "Are the Weak Interactions Strong?". Physics Letters B 101: 69. Bibcode 1981PhLB..101...69A. doi:10.1016/0370-2693(81)90492-5. [43] Bilson-Thompson, Sundance O.; Markopoulou, Fotini; Smolin, Lee (2007). "Quantum gravity and the standard model". Class. Quantum Grav. 24 (16): 3975–3993. arXiv:hep-th/0603022. Bibcode 2007CQGra..24.3975B. doi:10.1088/0264-9381/24/16/002. [44] K. G. Zloshchastiev, Spontaneous symmetry breaking and mass generation as built-in phenomena in logarithmic nonlinear quantum theory,

Acta Phys. Polon. B 42 (2011) 261-292 ArXiv:0912.4139 (http:/ / arxiv. org/ abs/ 0912. 4139). [45] A. V. Avdeenkov and K. G. Zloshchastiev, Quantum Bose liquids with logarithmic nonlinearity: Self-sustainability and emergence of

spatial extent, J. Phys. B: At. Mol. Opt. Phys. 44 (2011) 195303. ArXiv:1108.0847 (http:/ / arxiv. org/ abs/ 1108. 0847).

[46] Ian Sample (29 May 2009). "Anything but the God particle" (http:/ / www. guardian. co. uk/ science/ blog/ 2009/ may/ 29/ why-call-it-the-god-particle-higgs-boson-cern-lhc). London: The Guardian. . Retrieved 2009-06-24.

[47] Ian Sample (3 March 2009). "Father of the God particle: Portrait of Peter Higgs unveiled" (http:/ / www. guardian. co. uk/ science/ blog/

2009/ mar/ 02/ god-particle-peter-higgs-portrait-lhc). London: The Guardian. . Retrieved 2009-06-24.

[48] Randerson, James (June 30, 2008). "Father of the 'God Particle'" (http:/ / www. guardian. co. uk/ science/ 2008/ jun/ 30/ higgs. boson. cern). The Guardian. .

[49] Ian Sample (12 June 2009). "Higgs competition: Crack open the bubbly, the God particle is dead" (http:/ / www. guardian. co. uk/ science/

blog/ 2009/ jun/ 05/ cern-lhc-god-particle-higgs-boson). The Guardian (London). . Retrieved 2010-05-04.

References

Further reading • G.S. Guralnik, C.R. Hagen and T.W.B. Kibble (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13 (20): 585. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585. • G.S. Guralnik (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles". International Journal of Modern Physics A 24 (14): 2601–2627. arXiv:0907.3466. Bibcode 2009IJMPA..24.2601G. doi:10.1142/S0217751X09045431. • Guralnik, G S; Hagen, C R and Kibble, T W B (1967). Broken Symmetries and the Goldstone Theorem.

Advances in Physics, vol. 2 (http:/ / www. datafilehost. com/ download-7d512618. html) • F. Englert and R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13 (9): 321. Bibcode 1964PhRvL..13..321E. doi:10.1103/PhysRevLett.13.321. Higgs boson 42

• P. Higgs (1964). "Broken Symmetries, Massless Particles and Gauge Fields". Physics Letters 12 (2): 132. Bibcode 1964PhL....12..132H. doi:10.1016/0031-9163(64)91136-9. • P. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13 (16): 508. Bibcode 1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508. • P. Higgs (1966). "Spontaneous Symmetry Breakdown without Massless Bosons". Physical Review 145 (4): 1156. Bibcode 1966PhRv..145.1156H. doi:10.1103/PhysRev.145.1156. • Y. Nambu and G. Jona-Lasinio (1961). "Dynamical Model of Elementary Particles Based on an Analogy with Superconductivity". Physical Review 122: 345–358. Bibcode 1961PhRv..122..345N. doi:10.1103/PhysRev.122.345. • J. Goldstone, A. Salam and S. Weinberg (1962). "Broken Symmetries". Physical Review 127 (3): 965. Bibcode 1962PhRv..127..965G. doi:10.1103/PhysRev.127.965. • P.W. Anderson (1963). "Plasmons, Gauge Invariance, and Mass". Physical Review 130: 439. Bibcode 1963PhRv..130..439A. doi:10.1103/PhysRev.130.439. • A. Klein and B.W. Lee (1964). "Does Spontaneous Breakdown of Symmetry Imply Zero-Mass Particles?". Physical Review Letters 12 (10): 266. Bibcode 1964PhRvL..12..266K. doi:10.1103/PhysRevLett.12.266. • W. Gilbert (1964). "Broken Symmetries and Massless Particles". Physical Review Letters 12 (25): 713. Bibcode 1964PhRvL..12..713G. doi:10.1103/PhysRevLett.12.713.

External links

• Hunting the Higgs boson at C.M.S. Experiment, at CERN (http:/ / cmsinfo. web. cern. ch/ cmsinfo/ Physics/

HuntingHiggs/ index. html)

• The Higgs boson (http:/ / www. exploratorium. edu/ origins/ cern/ ideas/ higgs. html)" by the CERN exploratorium.

• Particle Data Group: Review of searches for Higgs bosons. (http:/ / pdg. lbl. gov/ 2005/ reviews/ contents_sports. html#hyppartetc) Higgs mechanism 43 Higgs mechanism

In particle physics, the Higgs mechanism is the process in which gauge bosons in a gauge theory can acquire non-vanishing masses through absorption of Nambu-Goldstone bosons arising in spontaneous symmetry breaking. The simplest implementation of the mechanism adds an extra Higgs field to the gauge theory. The spontaneous symmetry breaking of the underlying local symmetry triggers conversion of components of this Higgs field to Goldstone bosons which interact with (at least some of) the other fields in the theory, so as to produce mass terms for (at least some of) the gauge bosons. This mechanism may also leave behind elementary scalar (spin-0) particles, known as Higgs bosons. In the standard model, the phrase "Higgs mechanism" refers specifically to the generation of masses for the W±, and Z weak gauge bosons through electroweak symmetry breaking.[1] Although the evidence for the electroweak Higgs mechanism is overwhelming, experiments have yet to discover the single Higgs boson predicted by the standard model. The Large Hadron Collider at CERN is currently searching for Higgs bosons, and attempting to understand the electroweak Higgs mechanism.

In the standard model The Higgs mechanism was incorporated into modern particle physics by Steven Weinberg and Abdus Salam, and is an essential part of the standard model. In the standard model, at temperatures high enough so that electroweak symmetry is unbroken, all elementary particles are massless. At a critical temperature, the symmetry is spontaneously broken, and the W and Z bosons acquire masses. (EWSB, ElectroWeak Symmetry Breaking, is an abbreviation used for this). Fermions, such as the leptons and quarks in the Standard Model, can also acquire mass as a result of their interaction with the Higgs field, but not in the same way as the gauge bosons.

Structure of the Higgs field In the standard model, the Higgs field is an SU(2) doublet, a complex spinor with four real components (or equivalently with two complex components), with a Standard Model U(1) charge of −1. It transforms as a spinor under SU(2). Under U(1) rotations, it gets multiplied by a phase; this mixes the real and imaginary part of the complex spinor into each other, so this is not the same as two complex spinors mixing under U(1) (which would have eight real components between them), but instead is the spinor representation of the group U(2). The Higgs field, through the interactions specified by its potential, induces spontaneous breaking of three out of the four generators ("directions") of the gauge group SU(2)×U(1), and three out of its four components would ordinarily amount to Goldstone bosons, if they were not coupled to gauge fields. However, after symmetry breaking, these three of the four degrees of freedom in the Higgs field mix with the W and Z bosons, while the one remaining degree of freedom becomes the Higgs boson – a new scalar particle.

The part that remains massless The gauge group of the electroweak part of the standard model is SU(2) × U(1). The group SU(2) is all unitary matrices, all the orthonormal changes of coordinates in a complex two dimensional vector space. Rotating the coordinates so that the first basis vector in the direction of H makes the vacuum expectation value of H the spinor (A, 0). The generators for rotations about the x, y, and z axes are by half the Pauli matrices , so that a rotation of angle θ about the z-axis takes the vacuum to: Higgs mechanism 44

While the X and Y generators mix up the top and bottom components of the spinor, the Z rotations only multiply by a phase. This phase can be undone by a U(1) rotation of angle ½θ, which multiplies by the opposite phase, since the Higgs has charge −1. Under both an SU(2) z-rotation and a U(1) rotation by an amount ½θ, the vacuum is invariant. This combination of generators:

defines the unbroken gauge group, where W is the generator of rotations around the z-axis in the SU(2) and V is the z generator of the U(1). This combination of generators (a z rotation in the SU(2) and a simultaneous U(1) rotation by half the angle) preserves the vacuum, and defines the unbroken gauge group in the standard model. The part of the gauge field in this direction stays massless, and this gauge field is the actual photon. The phase that a field acquires under this combination of generators is its electric charge, and this is the formula for the electric charge in the standard model. In this convention, all the V charges in the standard model are multiples of ⅓. To make all the V-charges in the standard model integers, you can rescale the V part of the formula by tripling all the V-charges if you like, and rewrite the charge formula as:

but the normalization with ½V is the universal standard.

Consequences for fermions In spite of the introduction of spontaneous symmetry-breaking, also for fermions the mass terms oppose the chiral gauge invariance. Therefore, also for these fields the mass terms should be replaced by a gauge-invariant "Higgs" mechanism. An obvious possibility is some kind of "Yukawa coupling" (see below) between the fermion field ψ and the Higgs field Φ, with unknown couplings , which after symmetry-breaking (more precisely: after expansion of the Lagrange density around a suitable ground state) again results in the original mass terms, which are now, however (i.e. by introduction of the Higgs field) written in a gauge-invariant way. The Lagrange density for the "Yukawa"-interaction of a fermion field 'ψ' and the Higgs field 'Φ' is

where again the gauge field A only enters (i.e., it is only indirectly visible). The quantities are the Dirac matrices, and is the already-mentioned "Yukawa"-coupling parameter. Already now the mass-generation follows the same principle as above, namely from the existence of a finite expectation value , as described above. Again, this is crucial for the existence of the property "mass". History of research

Background Spontaneous symmetry breaking offered a framework to introduce bosons into relativistic quantum field theories. However, according to Goldstone's theorem, these bosons should be massless.[2] The only observed particles which could be approximately interpreted as Goldstone bosons were the pions, which Yoichiro Nambu related to chiral symmetry breaking. A similar problem arises with Yang–Mills theory (also known as nonabelian gauge theory), which predicts massless spin-1 gauge bosons. Massless weakly interacting gauge bosons lead to long-range forces, which are only observed for electromagnetism and the corresponding massless photon. Gauge theories of the weak force needed a way to describe massive gauge bosons in order to be consistent. Higgs mechanism 45

Discovery

The Higgs mechanism is also called the Brout–Englert–Higgs mechanism, or Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism,[3] or Anderson–Higgs mechanism. The mechanism was proposed in 1962 by Philip Warren Anderson,[4] who discussed its consequences for particle physics but did not work out an explicit relativistic model. The relativistic model was developed in 1964 by Peter Higgs,[5] and independently by Robert Brout and Francois Englert,[6] and Gerald Guralnik, C. R. Hagen, and Tom Five of the six 2010 APS Sakurai Prize Kibble,[7] who worked out the results by the spring of 1963.[8] The Winners- (L to R) Kibble, Guralnik, mechanism is closely analogous to phenomena previously discovered by Hagen, Englert, and Brout Yoichiro Nambu involving the "vacuum structure" of quantum fields in superconductivity.[9] A similar but distinct effect, known as the Stueckelberg mechanism, had previously been studied by Ernst Stueckelberg. These physicists discovered that when a gauge theory is combined with an additional field breaking spontaneously the symmetry group, the gauge bosons can consistently acquire a finite mass. In spite of the large values involved (see below) this permits a gauge theory description of the weak force, which was independently developed by Steven Weinberg and Abdus Salam in 1967. Higgs's original article presenting the model was rejected by Physics Letters. When revising the article before resubmitting it to Physical Number six: P. Higgs 2009 Review Letters, he added a sentence at the end,[10] mentioning that it implies the existence of one or more new, massive scalar bosons, which do not form complete representations of the symmetry group; these are the Higgs bosons.

The three papers by Guralnik, Hagen, and Kibble; Higgs; and Brout and Englert were each recognized as "milestone letters" by Physical Review Letters in 2008.[11] While each of these seminal papers took similar approaches, the contributions and differences among the 1964 PRL symmetry breaking papers are noteworthy. All six physicists were jointly awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics for this work.[12] Benjamin W. Lee is often credited with first naming the "Higgs-like" mechanism, although there is debate around when this first occurred.[13] [14] [15] One of the first times the Higgs name appeared in print was in 1972 when Gerardus 't Hooft and Martinus J. G. Veltman referred to it as the "Higgs-Kibble mechanism" in their Nobel winning paper.[16] [17]

Examples of Higgs mechanism The Higgs mechanism occurs whenever a charged field has a vacuum expectation value. In the nonrelativistic context, this is the Landau model of a charged Bose-Einstein condensate, also known as a superconductor. In the relativistic condensate, the condensate is a scalar field, and is relativistically invariant.

Landau Model The Higgs mechanism is a type of superconductivity which occurs in the vacuum. It occurs when all of space is filled with a sea of particles which are charged, or, in field language, when a charged field has a nonzero vacuum expectation value. Interaction with the quantum fluid filling the space prevents certain forces from propagating over long distances (as it does in a superconducting medium, e.g. in the Ginzburg–Landau theory). A superconductor expels all magnetic fields from its interior, a phenomenon known as the Meissner effect. This was mysterious for a long time, because it implies that electromagnetic forces somehow become short-range inside the superconductor. Contrast this with the behavior of an ordinary metal. In a metal, the conductivity shields electric Higgs mechanism 46

fields by rearranging charges on the surface until the total field cancels in the interior. But magnetic fields can penetrate to any distance, and if a magnetic monopole (an isolated magnetic pole) is surrounded by a metal the field can escape without collimating into a string. In a superconductor, however, electric charges move with no dissipation, and this allows for permanent surface currents, not just surface charges. When magnetic fields are introduced at the boundary of a superconductor, they produce surface currents which exactly neutralize them. The Meissner effect is due to currents in a thin surface layer, whose thickness, the London penetration depth, can be calculated from a simple model (the Ginzburg–Landau theory). This simple model, treats superconductivity as a charged Bose–Einstein condensate. Suppose that a superconductor contains bosons with charge q. The wavefunction of the bosons can be described by introducing a quantum field, ψ, which obeys the Schrödinger equation as a field equation (in units where , the Planck quantum divided by 2π, is replaced by 1):

The operator ψ(x) annihilates a boson at the point x, while its adjoint creates a new boson at the same point. The wavefunction of the Bose–Einstein condensate is then the expectation value ψ of ψ(x), which is a classical function that obeys the same equation. The interpretation of the expectation value is that it is the phase that one should give to a newly created boson so that it will coherently superpose with all the other bosons already in the condensate. When there is a charged condensate, the electromagnetic interactions are screened. To see this, consider the effect of a gauge transformation on the field. A gauge transformation rotates the phase of the condensate by an amount which changes from point to point, and shifts the vector potential by a gradient.

When there is no condensate, this transformation only changes the definition of the phase of ψ at every point. But when there is a condensate, the phase of the condensate defines a preferred choice of phase. The condensate wave function can be written as

where ρ is real amplitude, which determines the local density of the condensate. If the condensate were neutral, the flow would be along the gradients of θ, the direction in which the phase of the Schrödinger field changes. If the phase θ changes slowly, the flow is slow and has very little energy. But now θ can be made equal to zero just by making a gauge transformation to rotate the phase of the field. The energy of slow changes of phase can be calculated from the Schrödinger kinetic energy,

and taking the density of the condensate ρ to be constant,

Fixing the choice of gauge so that the condensate has the same phase everywhere, the electromagnetic field energy has an extra term,

When this term is present, electromagnetic interactions become short-ranged. Every field mode, no matter how long the wavelength, oscillates with a nonzero frequency. The lowest frequency can be read off from the energy of a long wavelength A mode, Higgs mechanism 47

This is a harmonic oscillator with frequency:

The quantity |ψ|2 (=ρ2) is the density of the condensate of superconducting particles. In an actual superconductor, the charged particles are electrons, which are fermions not bosons. So in order to have superconductivity, the electrons need to somehow bind into Cooper pairs. The charge of the condensate q is therefore twice the electron charge e. The pairing in a normal superconductor is due to lattice vibrations, and is in fact very weak; this means that the pairs are very loosely bound. The description of a Bose–Einstein condensate of loosely bound pairs is actually more difficult than the description of a condensate of elementary particles, and was only worked out in 1957 by Bardeen, Cooper and Schrieffer in the famous BCS theory.

Abelian Higgs Mechanism Gauge invariance means that certain transformations of the gauge field do not change the energy at all. If an arbitrary gradient is added to A, the energy of the field is exactly the same. This makes it difficult to add a mass term, because a mass term tends to push the field toward the value zero. But the zero value of the vector potential is not a gauge invariant idea. What is zero in one gauge is nonzero in another. So in order to give mass to a gauge theory, the gauge invariance must be broken by a condensate. The condensate will then define a preferred phase, and the phase of the condensate will define the zero value of the field in a gauge invariant way. The gauge invariant definition is that a gauge field is zero when the phase change along any path from parallel transport is equal to the phase difference in the condensate wavefunction. The condensate value is described by a quantum field with an expectation value, just as in the Landau–Ginzburg model. In order for the phase of the vacuum to define a gauge, the field must have a phase (also referred to as 'to be charged'). In order for a scalar field Φ to have a phase, it must be complex, or (equivalently) it should contain two fields with a symmetry which rotates them into each other. The vector potential changes the phase of the quanta produced by the field when they move from point to point. In terms of fields, it defines how much to rotate the real and imaginary parts of the fields into each other when comparing field values at nearby points. The only renormalizable model where a complex scalar field Φ acquires a nonzero value is the Mexican-hat model, where the field energy has a minimum away from zero.

This defines the following Hamiltonian:

The first term is the kinetic energy of the field. The second term is the extra potential energy when the field varies from point to point. The third term is the potential energy when the field has any given magnitude. This potential energy V(z, Φ) = λ • (|z|2 - Φ2)2 has a graph which looks like a Mexican hat, which gives the model its name. In particular, the minimum energy value is not at z = 0, but on the circle of points where the magnitude of z is Φ. Higgs mechanism 48

When the field Φ(x) is not coupled to electromagnetism, the Mexican-hat potential has flat directions. Starting in any one of the circle of vacua and changing the phase of the field from point to point costs very little energy. Mathematically, if

Higgs potential V. For a fixed value of λ the potential is presented against the real and imaginary parts of Φ. The Mexican-hat or champagne-bottle profile at the ground should be noted.

with a constant prefactor, then the action for the field θ(x), i.e., the "phase" of the Higgs field Φ(x), has only derivative terms. This is not a surprise. Adding a constant to θ(x) is a symmetry of the original theory, so different values of θ(x) cannot have different energies. This is an example of Goldstone's theorem: spontaneously broken continuous symmetries normally produce massless excitations. The Abelian Higgs model is the Mexican-hat model coupled to electromagnetism:

The classical vacuum is again at the minimum of the potential, where the magnitude of the complex field φ is equal to Φ. But now the phase of the field is arbitrary, because gauge transformations change it. This means that the field θ(x) can be set to zero by a gauge transformation, and does not represent any actual degrees of freedom at all. Furthermore, choosing a gauge where the phase of the vacuum is fixed, the potential energy for fluctuations of the vector field is nonzero. So in the abelian Higgs model, the gauge field acquires a mass. To calculate the magnitude of the mass, consider a constant value of the vector potential A in the x direction in the gauge where the condensate has constant phase. This is the same as a sinusoidally varying condensate in the gauge where the vector potential is zero. In the gauge where A is zero, the potential energy density in the condensate is the scalar gradient energy:

And this energy is the same as a mass term ½m2A2 where m = qΦ.

Nonabelian Higgs mechanism The Nonabelian Higgs model has the following action:

where now the nonabelian field A is contained in D and in the tensor components and (the relation between A and those components is well-known from the Yang–Mills theory). It is exactly analogous to the Abelian Higgs model. Now the field φ is in a representation of the gauge group, and the gauge covariant derivative is defined by the rate of change of the field minus the rate of change from parallel Higgs mechanism 49

transport using the gauge field A as a connection.

Again, the expectation value of Φ defines a preferred gauge where the vacuum is constant, and fixing this gauge, fluctuations in the gauge field A come with a nonzero energy cost. Depending on the representation of the scalar field, not every gauge field acquires a mass. A simple example is in the renormalizable version of an early electroweak model due to Julian Schwinger. In this model, the gauge group is SO(3) (or SU(2)--- there are no spinor representations in the model), and the gauge invariance is broken down to U(1) or SO(2) at long distances. To make a consistent renormalizable version using the Higgs mechanism, introduce a scalar field φa which transforms as a vector (a triplet) of SO(3). If this field has a vacuum expectation value, it points in some direction in field space. Without loss of generality, one can choose the z-axis in field space to be the direction that φ is pointing, and then the vacuum expectation value of φ is (0, 0, A), where A is a constant with dimensions of mass ( ). Rotations around the z-axis form a U(1) subgroup of SO(3) which preserves the vacuum expectation value of φ, and this is the unbroken gauge group. Rotations around the x and y-axis do not preserve the vacuum, and the components of the SO(3) gauge field which generate these rotations become massive vector mesons. There are two massive W mesons in the Schwinger model, with a mass set by the mass scale A, and one massless U(1) gauge boson, similar to the photon. The Schwinger model predicts magnetic monopoles at the electroweak unification scale, and does not predict the Z meson. It doesn't break electroweak symmetry properly as in nature. But historically, a model similar to this (but not using the Higgs mechanism) was the first in which the weak force and the electromagnetic force were unified.

Affine Higgs mechanism Ernst Stueckelberg discovered a version of the Higgs mechanism by analyzing the theory of quantum electrodynamics with a massive photon. Stueckelberg's model is a limit of the regular Mexican hat Abelian Higgs model, where the vacuum expectation value H goes to infinity and the charge of the Higgs field goes to zero in such a way that their product stays fixed. The mass of the Higgs boson is proportional to H, so the Higgs boson becomes infinitely massive and disappears. The vector meson mass is equal to the product eH, and stays finite. The interpretation is that when a U(1) gauge field does not require quantized charges, it is possible to keep only the angular part of the Higgs oscillations, and discard the radial part. The angular part of the Higgs field θ has the following gauge transformation law:

The gauge covariant derivative for the angle (which is actually gauge invariant) is:

In order to keep θ fluctuations finite and nonzero in this limit, θ should be rescaled by H, so that its kinetic term in the action stays normalized. The action for the theta field is read off from the Mexican hat action by substituting .

since eH is the gauge boson mass. By making a gauge transformation to set θ = 0, the gauge freedom in the action is eliminated, and the action becomes that of a massive vector field: Higgs mechanism 50

To have arbitrarily small charges requires that the U(1) is not the circle of unit complex numbers under multiplication, but the real numbers R under addition, which is only different in the global topology. Such a U(1) group is non-compact. The field θ transforms as an affine representation of the gauge group. Among the allowed gauge groups, only non-compact U(1) admits affine representations, and the U(1) of electromagnetism is experimentally known to be compact, since charge quantization holds to extremely high accuracy. The Higgs condensate in this model has infinitesimal charge, so interactions with the Higgs boson do not violate charge conservation. The theory of quantum electrodynamics with a massive photon is still a renormalizable theory, one in which electric charge is still conserved, but magnetic monopoles are not allowed. For nonabelian gauge theory, there is no affine limit, and the Higgs oscillations cannot be too much more massive than the vectors.

References [1] G. Bernardi, M. Carena, and T. Junk: "Higgs bosons: theory and searches", Reviews of Particle Data Group: Hypothetical particles and

Concepts, 2007, http:/ / pdg. lbl. gov/ 2008/ reviews/ higgs_s055. pdf

[2] Guralnik, G S; Hagen, C R and Kibble, T W B (1967). Broken Symmetries and the Goldstone Theorem. Advances in Physics, vol. 2 (http:/ /

www. datafilehost. com/ download-7d512618. html)

[3] Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia (http:/ / www. scholarpedia. org/ article/ Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism) [4] P. W. Anderson (1962). "Plasmons, Gauge Invariance, and Mass". Physical Review 130 (1): 439–442. Bibcode 1963PhRv..130..439A. doi:10.1103/PhysRev.130.439. [5] Peter W. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13 (16): 508–509. Bibcode 1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508. [6] F. Englert and R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13 (9): 321–323. Bibcode 1964PhRvL..13..321E. doi:10.1103/PhysRevLett.13.321. [7] G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13 (20): 585–587. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585. [8] Gerald S. Guralnik (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles". International Journal of Modern Physics A24 (14): 2601–2627. arXiv:0907.3466. Bibcode 2009IJMPA..24.2601G. doi:10.1142/S0217751X09045431. [9] Nambu, Y (1960). "Quasiparticles and Gauge Invariance in the Theory of Superconductivity". Physical Review 117 (3): 648–663. Bibcode 1960PhRv..117..648N. doi:10.1103/PhysRev.117.648. [10] Higgs, Peter (2007). "Prehistory of the Higgs boson". Comptes Rendus Physique 8 (9): 970–972. Bibcode 2007CRPhy...8..970H. doi:10.1016/j.crhy.2006.12.006

[11] Physical Review Letters – 50th Anniversary Milestone Papers (http:/ / prl. aps. org/ 50years/ milestones#1964)

[12] American Physical Society – J. J. Sakurai Prize Winners (http:/ / www. aps. org/ units/ dpf/ awards/ sakurai. cfm)

[13] Rochester's Hagen Sakurai Prize Announcement (http:/ / www. pas. rochester. edu/ urpas/ news/ Hagen_030708)

[14] C.R. Hagen discusses naming of Higgs Boson in 2010 Sakurai Prize Talk (http:/ / www. youtube. com/ watch?v=QrCPrwRBi7E&

feature=PlayList& p=BDA16F52CA3C9B1D& playnext_from=PL& index=9)

[15] Anything but the God particle by Ian Sample (http:/ / www. guardian. co. uk/ science/ blog/ 2009/ may/ 29/ why-call-it-the-god-particle-higgs-boson-cern-lhc) [16] G. 't Hooft and M. Veltman (1972). "Regularization and Renormalization of Gauge Fields". Nuclear Physics B 44 (1): 189–219. Bibcode 1972NuPhB..44..189T. doi:10.1016/0550-3213(72)90279-9.

[17] Regularization and Renormalization of Gauge Fields by t'Hooft and Veltman (PDF) (http:/ / igitur-archive. library. uu. nl/ phys/

2005-0622-155148/ 13877. pdf) Higgs mechanism 51

Further reading • Schumm, Bruce A. (2004) Deep Down Things. Johns Hopkins Univ. Press. Chpt. 9.

External links • Guralnik, G.S.; Hagen, C.R.; Kibble, T.W.B. (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13 (20): 585–87. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585.

• Mark D. Roberts (1999) " A Generalized Higgs Model. (http:/ / www. arXiv. org/ abs/ hep-th/ 9904080)"

• Sakurai Prize Videos (http:/ / www. youtube. com/ view_play_list?p=BDA16F52CA3C9B1D)

• In CERN Courier, Steven Weinberg reflects on spontaneous symmetry breaking (http:/ / cerncourier. com/ cws/

article/ cern/ 32522)

• Steven Weinberg on LHC (http:/ / www. youtube. com/ watch?v=Zl4W3DYTIKw)

• Steven Weinberg Praises Teams for Higgs Boson Theory. (http:/ / www. pas. rochester. edu/ urpas/ news/ Hagen_030708)

• Physical Review Letters – 50th Anniversary Milestone Papers. (http:/ / prl. aps. org/ 50years/ milestones#1964)

• Imperial College London on PRL 50th Anniversary Milestone Papers. (http:/ / www3. imperial. ac. uk/

newsandeventspggrp/ imperialcollege/ newssummary/ news_13-6-2008-12-42-20?newsid=38514)

• " Introducing the little Higgs. (http:/ / physicsworld. com/ cws/ article/ print/ 11353)" Physics World.

• Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia. (http:/ / www. scholarpedia. org/

article/ Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism)

• History of Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia. (http:/ / www.

scholarpedia. org/ article/ Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism_(history))

• The Hunt for the Higgs at Tevatron (http:/ / apps3. aps. org/ aps/ meetings/ april10/ roser. pdf)

• The Mystery of Empty Space (https:/ / www. youtube. com/ watch?v=Y-vKh_jKX7Q) on YouTube. A lecture with UCSD physicist Kim Griest (43 minutes). Large Hadron Collider 52 Large Hadron Collider

Large Hadron Collider (LHC)

LHC experiments

ATLAS A Toroidal LHC Apparatus

CMS Compact Muon Solenoid

LHCb LHC-beauty

ALICE A Large Ion Collider Experiment

TOTEM Total Cross Section, Elastic Scattering and Diffraction Dissociation

LHCf LHC-forward

MoEDAL Monopole and Exotics Detector At the LHC

LHC preaccelerators

p and Pb Linear accelerators for protons (Linac 2) and Lead (Linac 3)

(not marked) Proton Synchrotron Booster

PS Proton Synchrotron

SPS Super Proton Synchrotron

Hadron colliders

Intersecting Storage Rings CERN, 1971–1984

Super Proton Synchrotron CERN, 1981–1984

ISABELLE BNL, cancelled in 1983

Tevatron Fermilab, 1987–2011

Relativistic Heavy Ion Collider BNL, 2000–present

Superconducting Super Collider Cancelled in 1993

Large Hadron Collider CERN, 2009–present

Super Large Hadron Collider Proposed, CERN, 2019–

Very Large Hadron Collider Theoretical

The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature. Large Hadron Collider 53

The LHC lies in a tunnel 27 kilometres (17 mi) in circumference, as deep as 175 metres (574 ft) beneath the Franco-Swiss border near Geneva, Switzerland. This synchrotron is designed to collide opposing particle beams of either protons at an energy of 7 teraelectronvolts (7 TeV or 1.12 microjoules) per nucleon, or lead nuclei at an energy of 574 TeV (92.0 µJ) per nucleus (2.76 TeV per nucleon).[1] [2] The term hadron refers to particles composed of quarks. The Large Hadron Collider was built by the European Organization for Nuclear Research (CERN) with the intention of testing various predictions of high-energy physics, including testing for the existence of the hypothesized Higgs boson[3] and of the large family of new particles predicted by supersymmetry.[4] It was built in collaboration with over 10,000 scientists and engineers from over 100 countries, as well as hundreds of universities and laboratories.[5] On 10 September 2008, the proton beams were successfully circulated in the main ring of the LHC for the first time,[6] but 9 days later operations were halted due to an explosion involving helium gas.[7] [8] On 20 November 2009 they were successfully circulated again,[9] with the first recorded proton–proton collisions occurring 3 days later at the injection energy of 450 GeV per beam.[10] After the 2009 winter shutdown, the LHC was restarted and the beam was ramped up to 3.5 TeV per beam[11] (half its designed energy).[12] On 30 March 2010, the first planned collisions took place between two 3.5 TeV beams, a new world record for the highest-energy man-made particle collisions.[13] The LHC will continue to operate at half energy until the end of 2012; it will not run at full energy (7 TeV per beam) until 2014.[14]

Purpose

Physicists hope that the LHC will help answer some of the fundamental open questions in physics, concerning the basic laws governing the interactions and forces among the elementary objects, the deep structure of space and time, and in particular the intersection of quantum mechanics and general relativity, where current theories and knowledge are unclear or break down altogether. Data is also needed from high energy particle experiments to indicate which versions of scientific models are more likely to be correct - in particular to choose between the Standard Model and Higgsless models and to validate their predictions and allow further theoretical development. Many theorists expect new physics beyond the Standard A simulated event in the CMS detector, featuring Model to emerge at the TeV-scale, based on unsatisfactory properties the appearance of the Higgs boson of the Standard Model. Issues possibly to be explored by LHC collisions include:[15]

• Is the Higgs mechanism for generating elementary particle masses via electroweak symmetry breaking actually realised in nature?[16] It is expected that the collider will either demonstrate or rule out the existence of the elusive Higgs boson, thereby allowing physicists to determine whether the Standard Model or its Higgsless model alternatives are more likely to be correct.[17] [18] [19] • Is supersymmetry, an extension of the Standard Model and Poincaré symmetry, realised in nature, implying that all known particles have supersymmetric partners?[20] [21] [22] • Are there extra dimensions,[23] as predicted by various models based on string theory, and can we detect them?[24] • What is the nature of the dark matter that appears to account for 23% of the mass of the universe? Other open questions which may be explored using high energy particle collisions: • It is already known that electromagnetism and the weak nuclear force are just different manifestations of a single force called the electroweak force. The LHC may clarify whether the electroweak force and the strong Large Hadron Collider 54

nuclear force are similarly just different manifestations of one universal unified force, as predicted by various Grand Unification Theories. • Why is the fourth fundamental force (gravity) so many orders of magnitude weaker than the other three fundamental forces? See also Hierarchy problem. • Are there additional sources of quark flavour mixing, beyond those already predicted within the Standard Model? • Why are there apparent violations of the symmetry between matter and antimatter? See also CP violation. • What are the nature and properties of quark-gluon plasma, believed to have existed in the early universe and in certain compact and strange astronomical objects today? This will be investigated by heavy ion collisions in ALICE.

Design

The LHC is the world's largest and highest-energy particle accelerator.[1] [25] The collider is contained in a circular tunnel, with a circumference of 27 kilometres (17 mi), at a depth ranging from 50 to 175 metres (160 to 574 ft) underground. The 3.8-metre (12 ft) wide concrete-lined tunnel, constructed between 1983 and 1988, was formerly used to house the Large Electron–Positron Collider.[26] It crosses the border between Switzerland and France at four points, with most of it in France. A Feynman diagram of one way the Higgs boson Surface buildings hold ancillary equipment such as compressors, may be produced at the LHC. Here, two quarks each emit a W or Z boson, which combine to ventilation equipment, control electronics and refrigeration plants. make a neutral Higgs. The collider tunnel contains two adjacent parallel beam pipes that intersect at four points, each containing a proton beam, which travel in opposite directions around the ring. Some 1,232 dipole magnets keep the beams on their circular path, while an additional 392 quadrupole magnets are used to keep the beams focused, in order to maximize the chances of interaction between the particles in the four intersection points, where the two beams will cross. In total, over 1,600 superconducting magnets are installed, with most weighing over 27 tonnes. Approximately 96 tonnes of liquid helium is needed to keep the magnets, made of copper-clad niobium-titanium, at their operating temperature of 1.9 K (−271.25 °C), making the LHC the largest cryogenic facility in the world at liquid helium temperature.

Map of the Large Hadron Collider at CERN Large Hadron Collider 55

Once or twice a day, as the protons are accelerated from 450 GeV to 7 TeV, the field of the superconducting dipole magnets will be increased from 0.54 to 8.3 teslas (T). The protons will each have an energy of 7 TeV, giving a total collision energy of 14 TeV. At this energy the protons have a Lorentz factor of about 7,500 and move at about 0.999999991 c, or about 3 metres per second slower than the speed of light (c).[27] It will take less than 90 microseconds (μs) for a proton to travel once around the main ring – a speed of about 11,000 revolutions per second. Rather than continuous beams, the protons will Superconducting quadrupole electromagnets are be bunched together, into 2,808 bunches, so that interactions between used to direct the beams to four intersection points, where interactions between accelerated the two beams will take place at discrete intervals never shorter than 25 protons will take place. nanoseconds (ns) apart. However it will be operated with fewer bunches when it is first commissioned, giving it a bunch crossing interval of 75 ns.[28] The design luminosity of the LHC is 1034 cm−2s−1, providing a bunch collision rate of 40 MHz.[29]

Prior to being injected into the main accelerator, the particles are prepared by a series of systems that successively increase their energy. The first system is the linear particle accelerator LINAC 2 generating 50-MeV protons, which feeds the Proton Synchrotron Booster (PSB). There the protons are accelerated to 1.4 GeV and injected into the Proton Synchrotron (PS), where they are accelerated to 26 GeV. Finally the Super Proton Synchrotron (SPS) is used to further increase their energy to 450 GeV before they are at last injected (over a period of 20 minutes) into the main ring. Here the proton bunches are accumulated, accelerated (over a period of 20 minutes) to their peak 7-TeV energy, and finally circulated for 10 to 24 hours while collisions occur at the four intersection points.[30] The LHC physics program is mainly based on proton–proton collisions. However, shorter running periods, typically one month per year, with heavy-ion collisions are included in the program. While lighter ions are considered as well, the baseline scheme deals with lead ions[31] (see A Large Ion Collider Experiment). The lead ions will be first accelerated by the linear accelerator LINAC 3, and the Low-Energy Ion Ring (LEIR) will be used as an ion storage and cooler unit. The ions will then be further accelerated by the PS and SPS before being injected into LHC ring, where they will reach an energy of 2.76 TeV per nucleon (or 575 TeV per ion), higher than the energies reached by the Relativistic Heavy Ion Collider. The aim of the heavy-ion program is to investigate quark–gluon plasma, which existed in the early universe.

CMS detector for LHC Detectors

Six detectors have been constructed at the LHC, located underground in large caverns excavated at the LHC's intersection points. Two of them, the ATLAS experiment and the Compact Muon Solenoid (CMS), are large, general purpose particle detectors.[25] A Large Ion Collider Experiment (ALICE) and LHCb, have more specific roles and the last two, TOTEM and LHCf, are very much smaller and are for very specialized research. The BBC's summary of the main detectors is:[32] Large Hadron Collider 56

Detector Description

ATLAS one of two general purpose detectors. ATLAS will be used to look for signs of new physics, including the origins of mass and extra dimensions.

CMS the other general purpose detector will, like ATLAS, hunt for the Higgs boson and look for clues to the nature of dark matter.

ALICE is studying a "fluid" form of matter called quark–gluon plasma that existed shortly after the Big Bang.

LHCb equal amounts of matter and antimatter were created in the Big Bang. LHCb will try to investigate what happened to the "missing" antimatter.

Operational history The first beam was circulated through the collider on the morning of 10 September 2008.[32] CERN successfully fired the protons around the tunnel in stages, three kilometres at a time. The particles were fired in a clockwise direction into the accelerator and successfully steered around it at 10:28 local time.[33] The LHC successfully completed its first major test: after a series of trial runs, two white dots flashed on a computer screen showing the protons travelled the full length of the collider. It took less than one hour to guide the stream of particles around its inaugural circuit.[34] CERN next successfully sent a beam of protons in a counterclockwise direction, taking slightly longer at one and a half hours due to a problem with the cryogenics, with the full circuit being completed at 14:59. On 19 September 2008, a quench occurred in about 100 bending magnets in sectors 3 and 4, causing a loss of approximately six tonnes of liquid helium, which was vented into the tunnel, and a temperature rise of about 100 kelvin in some of the affected magnets. Vacuum conditions in the beam pipe were also lost.[35] Shortly after the incident CERN reported that the most likely cause of the problem was a faulty electrical connection between two magnets, and that – due to the time needed to warm up the affected sectors and then cool them back down to operating temperature – it would take at least two months to fix it.[36] Subsequently, CERN released a preliminary analysis of the incident on 16 October 2008,[37] and a more detailed one on 5 December 2008.[38] Both analyses confirmed that the incident was indeed initiated by a faulty electrical connection. A total of 53 magnets were damaged in the incident and were repaired or replaced during the winter shutdown.[39] In the original timeline of the LHC commissioning, the first "modest" high-energy collisions at a center-of-mass energy of 900 GeV were expected to take place before the end of September 2008, and the LHC was expected to be operating at 10 TeV by the time of the official inauguration on 21 October 2008.[40] However, due to the delay caused by the above-mentioned incident, the collider was not operational until November 2009.[41] Despite the delay, LHC was officially inaugurated on 21 October 2008, in the presence of political leaders, science ministers from CERN's 20 Member States, CERN officials, and members of the worldwide scientific community.[42] Most of 2009 was spent on repairs and reviews from the damage caused by the quench incident. On 20 November, low-energy beams circulated in the tunnel for the first time since the incident. The early part of 2010 saw the continued ramp-up of beam in energies and early physics experiments. On 30 March 2010, LHC set a record for high-energy collisions, by colliding proton beams at a combined energy level of 7 TeV. The attempt was the third that day, after two unsuccessful attempts in which the protons had to be "dumped" from the collider and new beams had to be injected.[43] The first proton run ended on 4 November 2010. A run with lead ions started on 8 November 2010, and ended on 6 December 2010.[44] This allowed the ALICE experiment to study matter under extreme conditions similar to those shortly after the Big Bang.[45] CERN has declared that the LHC will run through to the end of 2012, with a short technical stop at the end of 2011. The energy for 2011 will be 3.5 TeV per beam. In 2013 the LHC will go into a long shutdown to prepare for higher-energy running starting in 2014.[14] Large Hadron Collider 57

Timeline

Date Event

10 Sep 2008 CERN successfully fired the first protons around the entire tunnel circuit in stages.

19 Sep 2008 Magnetic quench occurred in about 100 bending magnets in sectors 3 and 4, causing a loss of approximately 6 tonnes of liquid helium.

30 Sep 2008 First "modest" high-energy collisions planned but postponed due to accident.

16 Oct 2008 CERN released a preliminary analysis of the accident.

21 Oct 2008 Official inauguration.

5 Dec 2008 CERN released detailed analysis.

[46] 20 Nov 2009 Low-energy beams circulated in the tunnel for the first time since the accident.

[10] 23 Nov 2009 First particle collisions in all four detectors at 450 GeV.

30 Nov 2009 LHC becomes the world's highest-energy particle accelerator achieving 1.18 TeV per beam, beating the Tevatron's previous record of [47] 0.98 TeV per beam held for eight years.

28 Feb 2010 The LHC continues operations ramping energies to run at 3.5 TeV for 18 months to two years, after which it will be shut down to [48] prepare for the 14 TeV collisions (7 TeV per beam).

30 Mar 2010 The two beams collided at 7 TeV (3.5 TeV per beam) in the LHC at 13:06 CEST, marking the start of the LHC research program.

8 Nov 2010 Start of the first run with lead ions.

6 Dec 2010 End of the run with lead ions. Shutdown until early 2011.

[49] 13 Mar 2011 Beginning of the 2011 run with proton beams.

21 Apr 2011 LHC becomes the world's highest-luminosity hadron accelerator achieving a peak luminosity of 4.67·1032 cm−2s−1, beating the [50] Tevatron's previous record of 4·1032 cm−2s−1 held for one year.

[51] 17 Jun 2011 The high luminosity experiments ATLAS and CMS reach 1 fb-1 of collected data.

23 Oct 2011 The high luminosity experiments ATLAS and CMS reach 5 fb-1 of collected data.

Findings CERN scientists estimate that, if the Standard Model is correct, a single Higgs boson may be produced every few hours. At this rate, it may take about two to three years to collect enough data to discover the Higgs boson unambiguously. Similarly, it may take one year or more before sufficient results concerning supersymmetric particles have been gathered to draw meaningful conclusions.[1] On the other hand, some extensions of the Standard Model predict additional particles, such as the heavy W' and Z' gauge bosons, whose existence might already be probed after a few months of data collection.[52] The first physics results from the LHC, involving 284 collisions which took place in the ALICE detector, were reported on 15 December 2009.[53] The results of the first proton–proton collisions at energies higher than Fermilab's Tevatron proton–antiproton collisions were published by the CMS collaboration in early February 2010, yielding greater-than-predicted charged-hadron production.[54] After the first year of data collection, the LHC experimental collaborations started to release their preliminary results concerning searches for new physics beyond the Standard Model in proton-proton collisions.[55] [56] [57] [58] No evidence of new particles was detected in the 2010 data. As a result, bounds were set on the allowed parameter space of various extensions of the Standard Model, such as models with large extra dimensions, constrained versions of the Minimal Supersymmetric Standard Model, and others.[59] [60] [61] Large Hadron Collider 58

On 24 May 2011 it was reported that quark–gluon plasma (the densest matter besides black holes) has been created in the LHC.[62] Between July and August 2011, results of searches for the Higgs boson and for exotic particles, based on the data collected during the first half of the 2011 run, were presented in conferences in Grenoble[63] and Mumbai.[64] In the latter conference it was reported that, despite hints of a Higgs signal in earlier data, ATLAS and CMS exclude with 95% confidence level the existence of a Higgs boson with the properties predicted by the Standard Model over most of the mass region between 145 and 466 GeV.[65] The searches for new particles did not yield signals either, allowing to further constrain the parameter space of various extensions of the Standard Model, including its supersymmetric extensions.[66] [67]

Proposed upgrade After some years of running, any particle physics experiment typically begins to suffer from diminishing returns: each additional year of operation discovers less than the year before. The way around the diminishing returns is to upgrade the experiment, either in energy or in luminosity. A luminosity upgrade of the LHC, called the Super LHC, has been proposed,[68] to be made in 2018 after ten years of operation. The optimal path for the LHC luminosity upgrade includes an increase in the beam current (i.e. the number of protons in the beams) and the modification of the two high-luminosity interaction regions, ATLAS and CMS. To achieve these increases, the energy of the beams at the point that they are injected into the (Super) LHC should also be increased to 1 TeV. This will require an upgrade of the full pre-injector system, the needed changes in the Super Proton Synchrotron being the most expensive. Currently the collaborative research effort of LHC Accelerator Research Program, LARP is conducting research into how to achieve these goals.[69]

Cost With a budget of 7.5 billion euros (approx. $9bn or £6.19bn as of Jun 2010), the LHC is one of the most expensive scientific instruments[70] ever built.[71] The total cost of the project is expected to be of the order of 4.6bn Swiss francs (approx. $4.4bn, €3.1bn, or £2.8bn as of Jan 2010) for the accelerator and SFr 1.16bn (approx. $1.1bn, €0.8bn, or £0.7bn as of Jan 2010) for the CERN contribution to the experiments.[72] The construction of LHC was approved in 1995 with a budget of SFr 2.6bn, with another SFr 210M towards the experiments. However, cost overruns, estimated in a major review in 2001 at around SFr 480M for the accelerator, and SFr 50M for the experiments, along with a reduction in CERN's budget, pushed the completion date from 2005 to April 2007.[73] The superconducting magnets were responsible for SFr 180M of the cost increase. There were also further costs and delays due to engineering difficulties encountered while building the underground cavern for the Compact Muon Solenoid,[74] and also due to faulty parts provided by Fermilab.[75] Due to lower electricity costs during the summer, it is expected that the LHC will normally not operate over the winter months,[76] although an exception was made to make up for the 2008 start-up delays over the 2009/10 winter.

Computing resources Data produced by LHC, as well as LHC-related simulation, was estimated at approximately 15 petabytes per year (max throughput while running not stated).[77] The LHC Computing Grid[78] was constructed to handle the massive amounts of data produced. It incorporated both private fiber optic cable links and existing high-speed portions of the public Internet, enabling data transfer from CERN to academic institutions around the world.[79] The Open Science Grid is used as the primary infrastructure in the United States, and also as part of an interoperable federation with the LHC Computing Grid. Large Hadron Collider 59

The distributed computing project LHC@home was started to support the construction and calibration of the LHC. The project uses the BOINC platform, enabling anybody with an Internet connection and either Windows, Mac OS X or Linux to use their computer's idle time to simulate how particles will travel in the tunnel. With this information, the scientists will be able to determine how the magnets should be calibrated to gain the most stable "orbit" of the beams in the ring.[80]

Safety of particle collisions The experiments at the Large Hadron Collider sparked fears among the public that the particle collisions might produce doomsday phenomena, involving the production of stable microscopic black holes or the creation of hypothetical particles called strangelets.[81] Two CERN-commissioned safety reviews examined these concerns and concluded that the experiments at the LHC present no danger and that there is no reason for concern,[82] [83] [84] a conclusion expressly endorsed by the American Physical Society.[85]

Operational challenges The size of the LHC constitutes an exceptional engineering challenge with unique operational issues on account of the amount of energy stored in the magnets and the beams.[30] [86] While operating, the total energy stored in the magnets is 10 GJ (equivalent to 2.4 tons of TNT) and the total energy carried by the two beams reaches 724 MJ (173 kilograms of TNT).[87] Loss of only one ten-millionth part (10−7) of the beam is sufficient to quench a superconducting magnet, while the beam dump must absorb 362 MJ (87 kilograms of TNT) for each of the two beams. These energies are carried by very little matter: under nominal operating conditions (2,808 bunches per beam, 1.15×1011 protons per bunch), the beam pipes contain 1.0×10−9 gram of hydrogen, which, in standard conditions for temperature and pressure, would fill the volume of one grain of fine sand.

Construction accidents and delays • On 25 October 2005, José Pereira Lages, a technician, was killed in the LHC when a switchgear that was being transported fell on him.[88] • On 27 March 2007 a cryogenic magnet support broke during a pressure test involving one of the LHC's inner triplet (focusing quadrupole) magnet assemblies, provided by Fermilab and KEK. No one was injured. Fermilab director Pier Oddone stated "In this case we are dumbfounded that we missed some very simple balance of forces". This fault had been present in the original design, and remained during four engineering reviews over the following years.[89] Analysis revealed that its design, made as thin as possible for better insulation, was not strong enough to withstand the forces generated during pressure testing. Details are available in a statement from Fermilab, with which CERN is in agreement.[90] [91] Repairing the broken magnet and reinforcing the eight identical assemblies used by LHC delayed the startup date, then planned for November 2007. • Problems occurred on 19 September 2008 during powering tests of the main dipole circuit, when an electrical fault in the bus between magnets caused a rupture and a leak of six tonnes of liquid helium. The operation was delayed for several months.[92] It is currently believed that a faulty electrical connection between two magnets caused an arc, which compromised the liquid-helium containment. Once the cooling layer was broken, the helium flooded the surrounding vacuum layer with sufficient force to break 10-ton magnets from their mountings. The explosion also contaminated the proton tubes with soot.[38] [93] This accident was thoroughly discussed in a 22 February 2010 Superconductor Science and Technology article by CERN physicist Lucio Rossi.[94] • Two vacuum leaks were identified in July 2009, and the start of operations was further postponed to mid-November 2009.[95] Large Hadron Collider 60

Popular culture The Large Hadron Collider gained a considerable amount of attention from outside the scientific community and its progress is followed by most popular science media. The LHC has also sparked the imaginations of authors of works of fiction, such as novels, TV series, and video games, although descriptions of what it is, how it works, and projected outcomes of the experiments are often only vaguely accurate, occasionally causing concern among the general public. The novel Angels & Demons, by Dan Brown, involves antimatter created at the LHC to be used in a weapon against the Vatican. In response CERN published a "Fact or Fiction?" page discussing the accuracy of the book's portrayal of the LHC, CERN, and particle physics in general.[96] The movie version of the book has footage filmed on-site at one of the experiments at the LHC; the director, Ron Howard, met with CERN experts in an effort to make the science in the story more accurate.[97] In The Big Bang Theory, the episode "The Large Hadron Collision" features the Large Hadron Collider prominently - albeit as a concept (the collider itself is not shown). The novel FlashForward, by Robert J. Sawyer, involves the search for the Higgs boson at the LHC. CERN published a "Science and Fiction" page interviewing Sawyer and physicists about the book and the TV series based on it.[98] CERN employee Katherine McAlpine's "Large Hadron Rap"[99] surpassed 6.7 million YouTube views.[100] [101] The band Les Horribles Cernettes was founded by female members of CERN. The name was chosen so to have the same initials as the LHC.[102] [103]

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MusiClub/ bands/ cernettes/ Press/ NYT. pdf). New York Times (New York, USA). . Retrieved 2010-09-21.

[103] Heather McCabe (10 February 1999). "Grrl Geeks Rock Out" (http:/ / musiclub. web. cern. ch/ MusiClub/ bands/ cernettes/ Press/ Wired. pdf). Wired News. . Retrieved 2010-09-21. Large Hadron Collider 64

External links

• Official website (http:/ / lhc. web. cern. ch/ lhc/ )

• Overview of the LHC at CERN's public webpage (http:/ / public. web. cern. ch/ public/ en/ LHC/ LHC-en. html)

• CERN Courier magazine (http:/ / www. cerncourier. com/ )

• CERN (https:/ / twitter. com/ cern) on Twitter

• CMS Experiment at CERN (https:/ / twitter. com/ CMSExperiment) on Twitter

• Unofficial CERN (https:/ / twitter. com/ LHCExperiment) on Twitter

• LHC Portal (http:/ / www. lhcportal. com/ ) Web portal

• Lyndon Evans and Philip Bryant (eds) (2008). "LHC Machine" (http:/ / www. iop. org/ EJ/ journal/ -page=extra.

lhc/ jinst). Journal of Instrumentation 3 (8): S08001. Bibcode 2008JInst...3S8001E. doi:10.1088/1748-0221/3/08/S08001. Full documentation for design and construction of the LHC and its six detectors (1600p).

• symmetry magazine LHC special issue August 2006 (http:/ / www. symmetrymagazine. org/ cms/ ?pid=1000350),

special issue December 2007 (http:/ / www. symmetrymagazine. org/ cms/ ?pid=1000562)

• New Yorker: Crash Course (http:/ / www. newyorker. com/ reporting/ 2007/ 05/ 14/ 070514fa_fact_kolbert). The world's largest particle accelerator.

• NYTimes: A Giant Takes On Physics' Biggest Questions (http:/ / www. nytimes. com/ 2007/ 05/ 15/ science/

15cern. html?ex=1336881600& en=7825f6702d7071e7& ei=5090& partner=rssuserland& emc=rss).

• Why a Large Hadron Collider? (http:/ / seedmagazine. com/ news/ 2006/ 07/ why_a_large_hadron_collider. php) Seed Magazine interviews with physicists.

• Thirty collected pictures during commissioning and post- 19 September 2008 incident repair (http:/ / www.

boston. com/ bigpicture/ 2009/ 11/ large_hadron_collider_ready_to. html), from Boston Globe.

• Podcast Interview (http:/ / omegataupodcast. net/ 2010/ 03/ 30-the-large-hadron-collider/ ) with CERN's Rolf Landua about the LHC and the physics behind it Peter Higgs 65 Peter Higgs

Peter Higgs

Born 29 May 1929 Newcastle upon Tyne, England

Nationality British

Fields Physics

Institutions University of Edinburgh Imperial College London University College London

Alma mater King's College London

Doctoral advisor Charles Coulson

Doctoral students Christopher Bishop Lewis Ryder David Wallace

Known for Broken symmetry in electroweak theory

Notable awards Wolf Prize in Physics (2004) Sakurai Prize (2010) Dirac Medal

Peter Ware Higgs, FRS, FRSE, FKC (born 29 May 1929), is an English theoretical physicist and an emeritus professor at the University of Edinburgh.[1] He is best known for his 1960s proposal of broken symmetry in electroweak theory, explaining the origin of mass of elementary particles in general and of the W and Z bosons in particular. This so-called Higgs mechanism, which had several inventors besides Higgs at about the same time, predicts the existence of a new particle, the Higgs boson (often described as "the most sought-after particle in modern physics"[2] [3] ). Although this particle has not turned up in accelerator experiments so far, the Higgs mechanism is generally accepted as an important ingredient in the Standard Model of particle physics, without which particles would have no mass.[4] Prof. Higgs has been honored with a number of awards in recognition of his work, including the 1997 Dirac Medal and Prize for outstanding contributions to theoretical physics from the Institute of Physics, the 1997 High Energy and Particle Physics Prize by the European Physical Society, the 2004 Wolf Prize in Physics, and the 2010 J. J. Sakurai Prize for Theoretical Particle Physics. Peter Higgs 66

Early life, education and career Higgs was born in Wallsend, Newcastle upon Tyne.[5] His father worked as a sound engineer for the BBC, and as a result of childhood asthma, together with the family moving around because of his father's job, and later the Second World War, Higgs missed some early schooling and was taught at home. When his father relocated to Bedford, Higgs stayed behind with his mother in Bristol, and was largely raised there. He attended that city's Cotham Grammar School,[6] where he was inspired by the work of one of the school's alumni, Paul Dirac, a founder of the field of quantum mechanics.[5] At the age of 17 Higgs moved to City of London School, where he specialized in mathematics, then to King's College London where he graduated with a first class honours degree in Physics, a masters degree, and Ph.D.[1] He became a Senior Research Fellow at the Edinburgh University, then held various posts at University College London and Imperial College London before becoming a temporary lecturer in Mathematics at University College London. He returned to Edinburgh University in 1960 to take up the post of Lecturer at the Tait Institute of Mathematical Physics, allowing him to settle in the city he had fallen in love with after hitch-hiking to the Edinburgh Fringe festival as a student.[1] Dr. Higgs was promoted to a personal chair of Theoretical Physics at Edinburgh in 1980. He became a fellow of the Royal Society in 1983, was awarded the Rutherford Medal and Prize in 1984, and became a fellow of the Institute of Physics in 1991. He retired in 1996 and became Emeritus professor at the University of Edinburgh.[1]

Work in theoretical physics At Edinburgh Higgs first became interested in mass, developing the idea that particles were massless when the universe began, acquiring mass a fraction of a second later as a result of interacting with a theoretical field (which became known as the Higgs field). Higgs postulated that this field permeates space, giving all elementary subatomic particles that interact with it their mass.[5] [7] While the Higgs field is postulated to confer mass on quarks and leptons, it causes only a tiny portion of the masses of other subatomic particles, such as protons and neutrons. In these, gluons that bind quarks together confer most of the particle mass. The original basis of Higgs' work came from the Japanese-born theorist and Nobel Prize winner Yoichiro Nambu from the University of Chicago. Professor Nambu had proposed a theory known as spontaneous symmetry breaking based on what was known to happen in superconductivity in condensed matter. However, the theory predicted massless particles (the Goldstone's theorem), a clearly incorrect prediction.[1] Higgs wrote a short paper exploiting a loophole in Goldstone's theorem and published it in Physics Letters, a European physics journal edited at CERN, in 1964.[8] Higgs wrote a second paper describing a theoretical model (now called the Higgs mechanism) but the paper was rejected (the editors of Physics Letters judged it "of no obvious relevance to physics"[5] ). Higgs wrote an extra paragraph and sent his paper to Physical Review Letters, another leading physics-journal, which published it later that year.[9] Other physicists, Robert Brout and Francois Englert[10] and Gerald Guralnik, C. R. Hagen, and Tom Kibble[11] had reached the same conclusion independently about the same time. The three papers written on this boson discovery by Higgs, Guralnik, Hagen, Kibble, Brout, and Englert were each recognized as milestone papers by Physical Review Letters 50th anniversary celebration.[12] While each of these famous papers took similar approaches, the contributions and differences between the 1964 PRL symmetry breaking papers are noteworthy. Nobelist Philip Anderson also claims to have "invented" the "Higgs" boson as far back as 1962. As an atheist, Higgs is reported to be displeased that the particle is nicknamed the "God particle". Higgs is afraid the term "might offend people who are religious".[13] [14] This nickname for the Higgs boson is usually attributed to Leon Lederman, but it is actually the result of Lederman's publisher's censoring. Originally Lederman intended to call it "the goddamn particle", because of its elusiveness.[15] Peter Higgs 67

The Large Hadron Collider at CERN in Switzerland detected possible signs of the Higgs boson in July 2011, a finding repeated by researchers at the Tevatron near Chicago. They reported "interesting particle events at a mass of between 140 and 145 gigaelectronvolts".[16] If the Higgs boson is found at CERN (ironically, home to the editor who famously rejected his initial paper), Higgs and the others who contributed to the Brout-Englert-Higgs-Guralnik-Hagen-Kibble mechanism may receive a Nobel Prize.[1]

Political views Higgs was a CND activist while in London and later in Edinburgh, but resigned his membership when the group extended its remit from campaigning against nuclear weapons to campaigning against nuclear power too.[17] [5] He was a Greenpeace member until the group opposed genetically modified organisms.[17] Higgs was awarded the 2004 Wolf Prize in Physics (sharing it with Brout and Englert), but he refused to fly to Jerusalem to receive the award because it was a state occasion attended by the then President of Israel, Moshe Katsav, and Higgs is opposed to Israel's actions in Palestine.[18]

Family Higgs has two sons: Chris, a computer scientist, and Jonny, a jazz musician.[13]

Cultural references A portrait of Peter Higgs was painted by Ken Currie in 2008.[19] Commissioned by the University of Edinburgh,[20] (subscription required) it was unveiled on 3 April 2009[21] and hangs in the entrance of the James Clerk Maxwell Building of the School of Physics and Astronomy.[19]

Notes and references [1] Griggs, Jessica. "The Missing Piece" from Edit the University of Edinburgh Alumni Magazine Summer 2008, Page 17

[2] Griffiths, Martin (20070501) physicsworld.com The Tale of the Blog's Boson (http:/ / physicsworld. com/ cws/ article/ print/ 27731) Retrieved on 2008-05-27

[3] Fermilab Today (20050616) Fermilab Results of the Week. Top Quarks are Higgs' best Friend (http:/ / www. fnal. gov/ pub/ today/

archive_2005/ today05-06-16. html) Retrieved on 2008-05-27

[4] Rincon, Paul (20040310) Fermilab 'God Particle' may have been seen (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 3546973. stm) Retrieved on 2008-05-27

[5] Sample, Ian. "The god of small things" (http:/ / www. guardian. co. uk/ weekend/ story/ 0,,2210858,00. html), The Guardian, November 17, 2007, weekend section.

[6] The Cotham Grammar School building now houses Cotham School, a specialist performing arts school. (http:/ / www. cotham. bristol. sch.

uk/ index. php?option=com_content& task=view& id=1& Itemid=1)

[7] "Higgs particle" (http:/ / www. britannica. com/ eb/ article-9040396/ Higgs-particle), Encyclopaedia Britannica, 2007. [8] P. W. Higgs Physics Letters 12 132 (1964). [9] P. W. Higgs Phys.Rev.Lett. 13 508 (1964).

[10] Broken Symmetry and the Mass of Gauge Vector Mesons (http:/ / prola. aps. org/ abstract/ PRL/ v13/ i9/ p321_1)

[11] Global Conservation Laws and Massless Particles (http:/ / prola. aps. org/ abstract/ PRL/ v13/ i20/ p585_1)

[12] Physical Review Letters - 50th Anniversary Milestone Papers (http:/ / prl. aps. org/ 50years/ milestones#1964)

[13] " Interview: the man behind the 'God particle' (http:/ / www. newscientist. com/ channel/ opinion/ mg19926732.

100-interview-the-man-behind-the-god-particle. html)", New Scientist 13 Sept., 2008, pp. 44–5

[14] Key scientist sure "God particle" will be found soon (http:/ / www. reuters. com/ article/ scienceNews/ idUSL0765287220080407?sp=true) Reuters news story. 7 April 2008.

[15] Randerson, James (June 30, 2008). "Father of the 'God Particle'" (http:/ / www. guardian. co. uk/ science/ 2008/ jun/ 30/ higgs. boson. cern). The Guardian. .

[16] Rincon, Paul (24 July 2011) "Higgs boson 'hints' also seen by US lab" (http:/ / www. bbc. co. uk/ news/ science-environment-14266358) BBC News. Retrieved 24 July 2011.

[17] Highfield, Roger. "Prof Peter Higgs profile" (http:/ / www. telegraph. co. uk/ science/ science-news/ 3338770/ Prof-Peter-Higgs-profile. html). The Telegraph. . Retrieved 16 May 2011. Peter Higgs 68

[18] Rodgers, Peter. "The heart of the matter" (http:/ / www. independent. co. uk/ news/ science/ the-heart-of-the-matter-558435. html). The Independent. . Retrieved 16 May 2011.

[19] "Portrait of Peter Higgs by Ken Currie, 2010" (http:/ / www. tait. ac. uk/ Peter_Higgs_by_Ken_Currie. html). The Tait Institute. . Retrieved 28 April 2011.

[20] Wade, Mike. "Portrait of a man at beginning of time" (http:/ / www. timesonline. co. uk/ tol/ news/ uk/ scotland/ article5835305. ece). The Times. . Retrieved 28 April 2011.

[21] "Great minds meet at portrait unveiling" (http:/ / www. ed. ac. uk/ news/ all-news/ higgs-portait-030309). The University of Edinburgh. . Retrieved 28 April 2011.

External links

• Peter Higgs (http:/ / genealogy. math. ndsu. nodak. edu/ id. php?id=35098) at the Mathematics Genealogy Project.

• Google Scholar (http:/ / scholar. google. com/ scholar?hl=en& lr=& q=author:PW+ author:Higgs& btnG=Search) List of Papers by PW Higgs

• A photograph of Peter Higgs (http:/ / www. particlephysics. ac. uk/ news/ picture-of-the-week/ picture-archive/

the-man-behind-the-higgs-particle. html), Photographs of Peter Higgs, June 2008 (http:/ / www. ph. ed. ac. uk/

peter-higgs/ )

• The god of small things (http:/ / www. guardian. co. uk/ science/ 2007/ nov/ 17/ sciencenews. particlephysics) - An interview with Peter Higgs in The Guardian

• Peter Higgs: the man behind the boson (http:/ / physicsweb. org/ articles/ world/ 17/ 7/ 6) - An article in the PhysicsWeb about Peter Higgs

• Higgs v Hawking: a battle of the heavyweights that has shaken the world of theoretical physics (http:/ / web.

archive. org/ web/ 20031116195026/ http:/ / millennium-debate. org/ ind3sept023. htm) - An article on the debate between Peter Higgs and Stephen Hawking about the existence of the Higgs boson

• My Life as a Boson (http:/ / wlap. physics. lsa. umich. edu/ umich/ mctp/ conf/ 2001/ sto2001/ higgs/ ) - A Lecture by Peter Higgs available in various formats

• blog of an interview (http:/ / theatomsmashers. blogspot. com/ 2004/ 07/ peter-higgs-as-in-higgs-boson. html)

• Physical Review Letters - 50th Anniversary Milestone Papers (http:/ / prl. aps. org/ 50years/ milestones#1964)

• In CERN Courier, Steven Weinberg reflects on spontaneous symmetry breaking (http:/ / cerncourier. com/ cws/

article/ cern/ 32522)

• Physics World, Introducing the little Higgs (http:/ / physicsworld. com/ cws/ article/ print/ 11353)

• Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia (http:/ / www. scholarpedia. org/

article/ Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism)

• History of Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia (http:/ / www. scholarpedia.

org/ article/ Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism_(history))

• Sakurai Prize Videos (http:/ / www. youtube. com/ view_play_list?p=BDA16F52CA3C9B1D) 69

Appendix

List of particles

This is a list of the different types of particles, known and hypothesized. For a chronological listing of subatomic particles by discovery date, see Timeline of particle discoveries. This is a list of the different types of particles found or believed to exist in the whole of the universe. For individual lists of the different particles, see the individual pages given below.

Elementary particles Elementary particles are particles with no measurable internal structure; that is, they are not composed of other particles. They are the fundamental objects of quantum field theory. Many families and sub-families of elementary particles exist. Elementary particles are classified according to their spin. Fermions have half-integer spin while bosons have integer spin. All the particles of the Standard Model have been observed, with the exception of the Higgs boson.

Fermions Fermions have half-integer spin; for all known elementary fermions this is 1⁄ . All known fermions are Dirac 2 fermions; that is, each known fermion has its own distinct antiparticle. It is not known whether the neutrino is a Dirac fermion or a Majorana fermion.[1] Fermions are the basic building blocks of all matter. They are classified according to whether they interact via the color force or not. In the Standard Model, there are 12 types of elementary fermions: six quarks and six leptons.

Quarks Quarks are the fundamental constituents of hadrons and interact via the strong interaction. Quarks are the only known carriers of fractional charge, but because they combine in groups of three (baryons) or in groups of two with antiquarks (mesons), only integer charge is observed in nature. Their respective antiparticles are the antiquarks which are identical except for the fact that they carry the opposite electric charge (for example the up quark carries charge +2⁄ , while the up antiquark carries charge −2⁄ ), color charge, and baryon number. There are six flavors of 3 3 quarks; the three positively charged quarks are called up-type quarks and the three negatively charged quarks are called down-type quarks.

Quarks

Name Symbol Antiparticle Charge Mass (MeV/c2) e

up u u +2⁄ 1.5–3.3 3

down d d −1⁄ 3.5–6.0 3

charm c c +2⁄ 1,160–1,340 3

strange s s −1⁄ 70–130 3

top t t +2⁄ 169,100–173,300 3 List of particles 70

bottom b b −1⁄ 4,130–4,370 3

Leptons Leptons do not interact via the strong interaction. Their respective antiparticles are the antileptons which are identical except for the fact that they carry the opposite electric charge and lepton number. The antiparticle of the electron is the antielectron, which is nearly always called positron for historical reasons. There are six leptons in total; the three charged leptons are called electron-like leptons, while the neutral leptons are called neutrinos.

Leptons

Name Symbol Antiparticle Charge Mass (MeV/c2) e

Electron e− e+ −1 0.511

Electron neutrino ν ν 0 e e

Muon μ− μ+ −1 105.7

Muon neutrino ν ν 0 < 0.170 μ μ

Tau τ− τ+ −1 1,777

Tau neutrino ν ν 0 < 15.5 τ τ

Bosons Bosons have integer spin. The fundamental forces of nature are mediated by gauge bosons, and mass is hypothesized to be created by the Higgs boson. According to the Standard Model (and to both linearized general relativity and string theory, in the case of the graviton) the elementary bosons are:

Name Symbol Antiparticle Charge (e) Spin Mass (GeV/c2) Interaction mediated Existence

Photon γ Self 0 1 0 Electromagnetism Confirmed

W boson W− W+ −1 1 80.4 Weak interaction Confirmed

Z boson Z Self 0 1 91.2 Weak interaction Confirmed

Gluon g Self 0 1 0 Strong interaction Confirmed

Higgs boson H0 Self 0 0 > 112 Mass Unconfirmed

Graviton G Self 0 2 0 Gravitation Unconfirmed

The graviton is added to the list although it is not predicted by the Standard Model, but by other theories in the framework of quantum field theory. The Higgs boson is postulated by electroweak theory primarily to explain the origin of particle masses. In a process known as the Higgs mechanism, the Higgs boson and the other fermions in the Standard Model acquire mass via spontaneous symmetry breaking of the SU(2) gauge symmetry. It is the only Standard Model particle not yet observed (the graviton is not a Standard Model particle). The Minimal Supersymmetric Standard Model (MSSM) predicts several Higgs bosons. If the Higgs boson exists, it is expected to be discovered at the Large Hadron Collider. List of particles 71

Hypothetical particles Supersymmetric theories predict the existence of more particles, none of which have been confirmed experimentally as of 2011:

Superpartners

Superpartner Superpartner Spin Notes of

neutralino neutral bosons 1⁄ The neutralinos are superpositions of the superpartners of neutral Standard Model bosons: neutral higgs 2 boson, Z boson and photon. The lightest neutralino is a leading candidate for dark matter. The MSSM predicts 4 neutralinos

chargino charged bosons 1⁄ The charginos are superpositions of the superpartners of charged Standard Model bosons: charged higgs 2 boson and W boson. The MSSM predicts two pairs of charginos.

photino photon 1⁄ Mixing with zino, neutral wino, and neutral Higgsinos for neutralinos. 2

wino, zino W± and Z0 1⁄ Charged wino mixing with charged Higgsino for charginos, for the zino see line above. 2 bosons

Higgsino Higgs boson 1⁄ For supersymmetry there is a need for several Higgs bosons, neutral and charged, according with their 2 superpartners.

gluino gluon 1⁄ Eight gluons and eight gluinos. 2

gravitino graviton 3⁄ Predicted by Supergravity (SUGRA). The graviton is hypothetical, too – see next table. 2 sleptons leptons 0 The superpartners of the leptons (electron, muon, tau) and the neutrinos.

sneutrino neutrino 0 Introduced by many extensions of the Standard Model, and may be needed to explain the LSND results. A special role has the sterile sneutrino, the supersymmetric counterpart of the hypothetical right-handed neutrino, called sterile neutrino

squarks quarks 0 The stop squark (superpartner of the top quark) is thought to have a low mass and is often the subject of experimental searches.

Note: Just as the photon, Z boson and W± bosons are superpositions of the B0, W0, W1, and W2 fields – the photino, zino, and wino± are superpositions of the bino0, wino0, wino1, and wino2 by definition. No matter if you use the original gauginos or this superpositions as a basis, the only predicted physical particles are neutralinos and charginos as a superposition of them together with the Higgsinos. Other theories predict the existence of additional bosons:

Other hypothetical bosons and fermions

Name Spin Notes

Higgs 0 Has been proposed to explain the origin of mass by the spontaneous symmetry breaking of the SU(2) x U(1) gauge symmetry. SUSY theories predict more than one type of Higgs boson

graviton 2 Has been proposed to mediate gravity in theories of quantum gravity.

graviscalar 0 Also known as radion

[2] graviphoton 1 Also known as gravivector

axion 0 A pseudoscalar particle introduced in Peccei-Quinn theory to solve the strong-CP problem. List of particles 72

axino 1⁄ Superpartner of the axion. Forms, together with the saxion and axion, a supermultiplet in supersymmetric extensions 2 of Peccei-Quinn theory.

saxion 0

branon ? Predicted in brane world models.

dilaton 0 Predicted in some string theories.

dilatino 1⁄ Superpartner of the dilaton 2 X and Y bosons 1 These leptoquarks are predicted by GUT theories to be heavier equivalents of the W and Z.

W' and Z' 1 bosons

magnetic photon ?

majoron 0 Predicted to understand neutrino masses by the seesaw mechanism.

majorana 1⁄ ; 3⁄ Gluinos, neutralinos, or other 2 2 fermion ?...

Mirror particles are predicted by theories that restore parity symmetry. Magnetic monopole is a generic name for particles with non-zero magnetic charge. They are predicted by some GUTs. Tachyon is a generic name for hypothetical particles that travel faster than the speed of light and have an imaginary rest mass. Preons were suggested as subparticles of quarks and leptons, but modern collider experiments have all but ruled out their existence. Kaluza-Klein towers of particles are predicted by some models of extra dimensions. The extra-dimensional momentum is manifested as extra mass in four-dimensional space-time.

Composite particles

Hadrons Hadrons are defined as strongly interacting composite particles. Hadrons are either: • Composite fermions, in which case they are called baryons. • Composite bosons, in which case they are called mesons. Quark models, first proposed in 1964 independently by Murray Gell-Mann and George Zweig (who called quarks "aces"), describe the known hadrons as composed of valence quarks and/or antiquarks, tightly bound by the color force, which is mediated by gluons. A "sea" of virtual quark-antiquark pairs is also present in each hadron. List of particles 73

Baryons (fermions)

For a detailed list, see List of baryons. Ordinary baryons (composite fermions) contain three valence quarks or three valence antiquarks each. • Nucleons are the fermionic constituents of normal atomic nuclei: • Protons, composed of two up and one down quark (uud) • Neutrons, composed of two down and one up quark (ddu) • Hyperons, such as the Λ, Σ, Ξ, and Ω particles, which contain one or more strange quarks, are short-lived and heavier than A combination of three u, d or s-quarks with a nucleons. Although not normally present in atomic nuclei, they total spin of 3⁄ form the so-called baryon 2 can appear in short-lived hypernuclei. decuplet. • A number of charmed and bottom baryons have also been observed. Some hints at the existence of exotic baryons have been found recently; however, negative results have also been reported. Their existence is uncertain. • Pentaquarks consist of four valence quarks and one valence antiquark.

Proton quark structure: 2 up quarks and 1 down quark.

Mesons (bosons)

For a detailed list, see List of mesons. Ordinary mesons are made up of a valence quark and a valence antiquark. Because mesons have spin of 0 or 1 and are not themselves elementary particles, they are composite bosons. Examples of mesons include the pion, kaon, the J/ψ. In quantum hadrodynamic models, mesons mediate the residual strong force between nucleons. At one time or another, positive signatures have been reported for all of the following exotic mesons but their existence has yet to be confirmed. Mesons of spin 0 form a nonet • A tetraquark consists of two valence quarks and two valence antiquarks; • A glueball is a bound state of gluons with no valence quarks; • Hybrid mesons consist of one or more valence quark-antiquark pairs and one or more real gluons. List of particles 74

Atomic nuclei

Atomic nuclei consist of protons and neutrons. Each type of nucleus contains a specific number of protons and a specific number of neutrons, and is called a nuclide or isotope. Nuclear reactions can change one nuclide into another. See table of nuclides for a complete list of isotopes.

Atoms

Atoms are the smallest neutral particles into which matter can be divided by chemical reactions. An atom consists of a small, heavy nucleus surrounded by a relatively large, light cloud of electrons. Each type of atom corresponds to a specific chemical element. To date, 118 elements have been discovered, while only the first 112 have received official names. Refer to the periodic table for an overview. A semi-accurate depiction of the helium atom. In the nucleus, the protons are in red and neutrons are in purple. In reality, the nucleus is also spherically symmetrical. The atomic nucleus consists of protons and neutrons. Protons and neutrons are, in turn, made of quarks.

Molecules

Molecules are the smallest particles into which a non-elemental substance can be divided while maintaining the physical properties of the substance. Each type of molecule corresponds to a specific chemical compound. Molecules are a composite of two or more atoms. See list of compounds for a list of molecules.

Condensed matter The field equations of condensed matter physics are remarkably similar to those of high energy particle physics. As a result, much of the theory of particle physics applies to condensed matter physics as well; in particular, there are a selection of field excitations, called quasi-particles, that can be created and explored. These include: • Phonons are vibrational modes in a crystal lattice. • Excitons are bound states of an electron and a hole. • Plasmons are coherent excitations of a plasma. • Polaritons are mixtures of photons with other quasi-particles. • Polarons are moving, charged (quasi-) particles that are surrounded by ions in a material. • Magnons are coherent excitations of electron spins in a material. List of particles 75

Other • An anyon is a generalization of fermion and boson in two-dimensional systems like sheets of graphene which obeys braid statistics. • A plekton is a theoretical kind of particle discussed as a generalization of the braid statistics of the anyon to dimension > 2. • A WIMP (weakly interacting massive particle) is any one of a number of particles that might explain dark matter (such as the neutralino or the axion). • The pomeron, used to explain the elastic scattering of Hadrons and the location of Regge poles in Regge theory. • The skyrmion, a topological solution of the pion field, used to model the low-energy properties of the nucleon, such as the axial vector current coupling and the mass. • A genon is a particle existing in a closed timelike world line where spacetime is curled as in a Frank Tipler or Ronald Mallett time machine. • A goldstone boson is a massless excitation of a field that has been spontaneously broken. The pions are quasi-Goldstone bosons (quasi- because they are not exactly massless) of the broken chiral isospin symmetry of quantum chromodynamics. • A goldstino is a Goldstone fermion produced by the spontaneous breaking of supersymmetry. • An instanton is a field configuration which is a local minimum of the Euclidean action. Instantons are used in nonperturbative calculations of tunneling rates. • A dyon is a hypothetical particle with both electric and magnetic charges • A geon is an electromagnetic or gravitational wave which is held together in a confined region by the gravitational attraction of its own field energy. • An inflaton is the generic name for an unidentified scalar particle responsible for the cosmic inflation. • A spurion is the name given to a "particle" inserted mathematically into an isospin-violating decay in order to analyze it as though it conserved isospin. • What is called "true muonium", a bound state of a muon and an antimuon, is a theoretical exotic atom which has never been observed.

Classification by speed • A tardyon or bradyon travels slower than light and has a non-zero rest mass. • A luxon travels at the speed of light and has no rest mass. • A tachyon (mentioned above) is a hypothetical particle that travels faster than the speed of light and has an imaginary rest mass.

References [1] B. Kayser, Two Questions About Neutrinos, arXiv:1012.4469v1 [hep-ph] (2010).

[2] R. Maartens (2004). Brane-World Gravity (http:/ / www. emis. de/ journals/ LRG/ Articles/ lrr-2004-7/ download/ lrr-2004-7BW. pdf). 7. 7. .

Also available in web format at http:/ / www. livingreviews. org/ lrr-2004-7. • C. Amsler et al. (Particle Data Group) (2008). "Review of Particle Physics". Physics Letters B 667 (1-5): 1. Bibcode 2008PhLB..667....1P. doi:10.1016/j.physletb.2008.07.018. (All information on this list, and more, can be

found in the extensive, biannually-updated review by the Particle Data Group (http:/ / pdg. lbl. gov)) Article Sources and Contributors 76 Article Sources and Contributors

Particle physics Source: http://en.wikipedia.org/w/index.php?oldid=461842377 Contributors: 128.12.93.xxx, 142.58.249.xxx, 64.26.98.xxx, APH, AgadaUrbanit, Agerom, Ahoerstemeier, Aknochel, Alansohn, Allstarecho, Almostcrime, AndreasJS, Andycjp, Archer7, Ark, Aroodman, Arthena, Atlant, Austin Maxwell, Awmarcz, AxelBoldt, Bambaiah, Bamkin, Barbara Shack, Battlemage, Bdesham, Bennylin, Bevo, Bggoldie, Bm gub, BobertWABC, Bobo192, Bodhitha, Boing! said Zebedee, Boud, Brandonlovescrashincastles, Brews ohare, BurtPeck, CRGreathouse, CRKingston, CWii, CYD, Calmypal, Caltas, CambridgeBayWeather, Can't sleep, clown will eat me, Celithemis, Ch2pgj, Chenyu, CimanyD, Cjc38, CloudNineAC, Complex (de), Comrade42, Conversion script, Csgwon, Cybercobra, DHN, DV8 2XL, Dauto, Deglr6328, Dev 176, Diligent Terrier, Discospinster, Djegan, Docu, Dominick, Donarreiskoffer, Donzzz77, Drphilharmonic, Ebehn, Edward Z. Yang, El C, El Snubbe, Ellywa, Elodzinski, Eloquence, EmanCunha, Emijrp, Eog1916, Erwinrossen, FT2, Falcon8765, Falconkhe, Favonian, Fieldday-sunday, Fruge, Gaius Cornelius, Gareth Owen, Gary King, Gbrandt, GeorgeLouis, Ghalhud, Giftlite, Glenn, Gnixon, Gnomon Kelemen, Goodnightmush, Goudzovski, Graham87, Haxwell, Hdeasy, Head, Headbomb, Hectorthebat, Henry W. Schmitt, Hepforever, Hfastedge, Howdychicken, Howie Goodell, IOPhysics, Ilmari Karonen, Immunize, Inwind, Iridescent, Ironboy11, Isnow, Ixfd64, J.delanoy, JRR Trollkien, JaGa, Jagged 85, Jameskeates, Jamesontai, JamieS93, Jgwacker, JimVC3, Jimbill4321, Joe N, Joe iNsecure, Jomoal99, JonasRH, Joshmt, Jpowell, Jung dalglish, Jxzj, Kakofonous, Karol Langner, Kbrose, Kenneth M Burke, Khcf6971, Kocio, Korath, Kozuch, Kuru, Kurzon, LX, Langsytank, Larry Sanger, Laussy, Le sacre, Lee Daniel Crocker, Lightdarkness, Lightmouse, Ling.Nut, Lmatt, Looxix, Lor772, Lseixas, Lumidek, Lupin, MER-C, MK8, Mako098765, Marcus Qwertyus, Master Jay, Mato, Matt Crypto, Matt Gies, Matthew Woodcraft, Mattmartin, Maurreen, Mav, Mayumashu, Mcneile, Md7t, Melchoir, Mermaid from the Baltic Sea, Metrictensor, Mets501, Michael Hardy, MichaelMaggs, Micraboy, Mignon, Mike2vil, MonoAV, MoogleEXE, Mouse7525, Mpatel, Mullactalk, Munkay, Mxn, Navasj, NellieBly, News0969, Novacatz, NuclearWinner, Ohconfucius, Ohwilleke, Oldnoah, Olexandr Kravchuk, Olhp, OpenToppedBus, Orion11M87, Orpheus, Oxymoron83, Paine Ellsworth, Palfrey, Pandacomics, Party, Patrick, Pcd72, Pchapman47879, Penarestel, Pet3r, PhoenixFlentge, PhySusie, Phys, Physics, Physicsdavid, Physis, Planlips, Plastadity, Poopfacer, PranksterTurtle, QFT, RE, Ragesoss, Rangoon11, Raphtee, Raul654, Ravi12346, Rclsa, Redvers, Res2216firestar, Rholton, Rich Farmbrough, Rje, Rl, Roadrunner, Rorro, Ryan Postlethwaite, SCZenz, Saeed.Veradi, SaltyBoatr, Sanders muc, SarahLawrence Scott, Savidan, Scottfisher, Selkem, Shawn in Montreal, Silly rabbit, SimonMayer, SimonP, Sjakkalle, Smarcus, Snigbrook, Sodium, Someguy1221, Sonicology, Srleffler, Stephenb, Steve Quinn, SwordSmurf, TallMagic, Techraj, The Epopt, Tpbradbury, Trecool12, Trelvis, Truthnlove, Tycho, UncleDouggie, UninvitedCompany, Urvabara, Van helsing, VanishedUser314159, Velella, VictorFlaushenstein, Vishnava, Voidxor, Voyajer, Wavelength, Who, WikHead, Will Gladstone, Witguiota, Wolftengu, XJamRastafire, Ylai, Zanzerjewel, Ъыь, 404 anonymous edits

Standard Model Source: http://en.wikipedia.org/w/index.php?oldid=462915508 Contributors: A. di M., APH, Adam Krellenstein, Addshore, Afteread, AgadaUrbanit, Agasicles, Agasides, Aknochel, Alan Liefting, Alansohn, Alinor, Aliotra, Alison, AmarChandra, Andre Engels, Andycjp, AnonMoos, Aoosten, Arivero, AugPi, Awren, AxelBoldt, Axl, Bakken, Bambaiah, Bamkin, Barak Sh, Bassbonerocks, Bdijkstra, BenRG, Benbest, Bender235, Beta Orionis, Bevo, Bodhitha, Bookalign, Bovineone, Brews ohare, Brim, Brockert, Bryan Derksen, Bubba73, Bytbox, C0nanPayne, CYD, Caco de vidro, CattleGirl, Chris the speller, ChristopherWillis, Complexica, Craig Bolon, Crazz bug 5, Crum375, Cybercobra, D-Notice, DWHalliday, DadaNeem, Daniel.Cardenas, DannyWilde, Dauto, Dave1g, David Barnard, David Schaich, David spector, Dbenbenn, Dbraize, Deepmath, DerNeedle, Derek Ross, Dextrose, Dfan, Diagramma Della Verita, Djr32, Dmmaus, Dratman, Drhex, Drrngrvy, Drxenocide, Dstudent, Dv82matt, Dysepsion, Edsegal, Edward, Eeekster, Egg, Ekjon Lok, El C, Elsweyn, Epbr123, Ernsts, Escalona, FT2, Faethon, Faethon34, Faethon36, Fences and windows, Fogger, FrankTobia, Gary King, Gatortpk, Geremia, Giftlite, Glenn, Gnixon, Goop Goop, Goudzovski, Gparker, Gscshoyru, Guarracino, Guy Harris, H2g2bob, HEL, Hal peridol, Hans Dunkelberg, Haoherb428, Harp, Harrigan, Headbomb, Herbee, Hexane2000, Hirak 99, HorsePunchKid, HungarianBarbarian, IanOfNorwich, Icairns, Icalanise, Iomesus, Isis, Isocliff, Itinerant1, J Milburn, J.delanoy, JLaTondre, JabberWok, Jacksonwalters, Jagged 85, JamesAM, JarahE, JeffBobFrank, Jeffq, Jeodesic, Jessemv, Jgwacker, Jim E. Black, Jmnbatista, Joshmt, Jrf, Jrtayloriv, [email protected], JulesH, Julesd, Kacser, Kate, KathrynLybarger, Kenmint, Kocio, Laurascudder, LeYaYa, Len Raymond, Leszek Jańczuk, Likebox, LilHelpa, Linas, Lomn, Looxix, Lottamiata, MJ94, Macumba, Maldmac, Mastertek, Melchoir, Metacomet, Michael C Price, Michael Hardy, Michaelbusch, Mindmatrix, Mjamja, Monedula, Moose-32, Mosaffa, MovGP0, Mpatel, Mxn, Naraht, Nozzer42, Nurg, Ohwilleke, Ordovico, Orion11M87, Orionus, Paine Ellsworth, Patrick, Patrickwooldridge, Pharotic, Phatom87, Phr, Phys, Physicist brazuca, Physics therapist, Populus, QFT, QMarion II, Qwertyca, R.e.b., RG2, Ram-Man, Rama, Raven in Orbit, Rbj, Reddi, Rjwilmsi, Roadrunner, Robdunst, Roscoe x, SCZenz, Schucker, Seaphoto, SebastianHelm, Securiger, Setanta747 (locked), Setreset, SheepNotGoats, Sheliak, Silly rabbit, Sligocki, Soarhead77, Sonjaaa, Stannered, Stephen Poppitt, Steve Quinn, Stevertigo, Stillnotelf, Stormymountain, Superm401, Superwj5, Suslindisambiguator, Swamy g, TPickup, Tanner Swett, Tarcieri, Tariqhada, Template namespace initialisation script, TenOfAllTrades, Tetracube, Texture, That Guy, From That Show!, The Anome, The Transliterator, Thunderboltz, TimBentley, Tirebiter78, Tom Lougheed, TriTertButoxy, Truthnlove, Twas Now, UnitedStatesian, UniversumExNihilo, Van helsing, Verdy p, Vessels42, Voorlandt, VoxMoose, WJBscribe, Waggers, Wilhelm-physiker, Wing gundam, Wtmitchell, Wwheaton, Xerxes314, Xezbeth, YellowMonkey, Yevgeny Kats, Youandme, ZakMarksbury, 332 anonymous edits

Supersymmetry Source: http://en.wikipedia.org/w/index.php?oldid=463089994 Contributors: Acjohnson55, Aknochel, Ancheta Wis, Andre Engels, Anville, Arivero, Barak Sh, BenRG, Bhny, Blaxthos, Bodera, Bryan Derksen, C9, CES1596, Cadmasteradam, Can't sleep, clown will eat me, Cgingold, Chaos, Charles Matthews, Charleswestbrook, Chessmaster7m, Cless Alvein, Closedmouth, Complexica, Crum375, Cuboidal, DO'Neil, Dan Gluck, Ddimensões, Deglr6328, Djloststylez, Drrngrvy, Duk, Eddie Nixon, Edward, El C, Epolk, F Notebook, Ferkelparade, Francescog, Fropuff, Gagoga ju, Gary King, Giftlite, Gil987, Girl Scout cookie, Gparker, Gsard, Gus Polly, HaloStereo1, Headbomb, IMSoP, Isocliff, J.christianson, JarahE, Jcpilman, Jeandré du Toit, Jgwacker, Jordan14, Josiah Rowe, Jpod2, KFP, Kawakameha, Kborland, Kevin Hickerson, Killing Vector, Koeplinger, Kostisl, Kurochka, Lambiam, LiDaobing, Lmatt, LostLeviathan, Lumidek, MFH, Maarten van Vliet, Maliz, Mastertek, Maurice Carbonaro, Maury Markowitz, Maxim Leyenson, Maxim Razin, Maximus Rex, Mdanziger, Mgnbar, Michael C Price, Michael Hardy, Mira, Mishas42, Monedula, Mor, Moyogo, Mpatel, Mporter, Nn123645, Nonnormalizable, Nowhither, Ohwilleke, Pearle, Pharotic, Phys, PhysPhD, Plumpurple, Ptrslv72, Puzl bustr, QFT, R.e.b., RG2, RJFJR, Radagast83, Raul654, Reaverdrop, Rich Farmbrough, Rjwilmsi, Roadrunner, Robma, Roybb95, Ruff ilb, Rursus, Salgueiro, Sam Hocevar, Scrabby, SeventyThree, Sheliak, Smack, Solarapex, Stevertigo, Susy is it, Taw, Ted BJ, That Guy, From That Show!, TheMaster42, Theresa knott, TimothyRias, Tktktk, TriTertButoxy, Tweet Tweet, Unconcerned, VermillionBird, Vyroglyph, Wangjiaji, Wavelength, WikHead, Wtmitchell, Xerxes314, Xiaphias, Yevgeny Kats, Zahd, Zentropa77, 197 anonymous edits

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Boson Source: http://en.wikipedia.org/w/index.php?oldid=458547350 Contributors: 217.126.156.xxx, Abdullais4u, Abtvctkto61, Ace of Spades, Aldie, Altenmann, Alxndr, Andres, Andrewa, Anonymous Dissident, Anthony Appleyard, Antixt, Asnr 6, Beland, Ben-Zin, Benbest, Bertrem, Bkalafut, Bradenripple, BriEnBest, Buckyboy314, CAkira, CYD, Candamir, Cenarium, Complexica, Conversion script, CosineKitty, Cructacean, Dan Gluck, DanielCD, Danjel, Danthewhale, Dicklyon, Discospinster, Drxenocide, Enchanter, Enormousdude, Epastore, Evil saltine, Fropuff, Frozenport, Giftlite, Gilliam, Glenn, Gravitivistically, GregorB, H2g2bob, Hadal, Hal peridol, Headbomb, Hidaspal, Hqb, Ianji, Icairns, Inertiatic076, Iridescent, Jack who built the house, Jensbn, Jeraaldo, Jim E. Black, Jlandahl, Jossi, Kaihsu, Kbdank71, Kbh3rd, Kingpin13, La goutte de pluie, Leoadec, Lisiate, Looxix, MK8, MagnaMopus, Master Justin, Mathninja, Mbell, Merovingian, Mgurgan, Michael Hardy, MichaelMaggs, Mike2vil, Minami Kana, Mormegil, Mpatel, Nakos2208, NawlinWiki, Nilradical, Noisy, Nsajjansajja, OlEnglish, Orionus, Ornil, Ouchitburns, Owhanow, Palica, Panoramix, Pharotic, Philvarner, Phys, Pkoppenb, Pyg, QFT, R'n'B, RadicalOne, Radon210, Roadrunner, Robhenry9, Robinh, Rocastelo, Rorro, RuneSylvester, Salsb, Schneelocke, Scienz Guy, Senator Palpatine, Srleffler, Stebbins, Strait, Sunborn, T-dot, Tarotcards, The Anome, The Wild Falcon, The1physicist, TheEditrix2, Tim Starling, Tpvibes, Uberdude85, UltraHighVacuum, VashiDonsk, Voyajer, W.F.Galway, Ward3001, Welsh, WhiteHatLurker, Wikeepedian, Wikiborg, Wragge, Zfr, Zzedar, 158 anonymous edits

Higgs boson Source: http://en.wikipedia.org/w/index.php?oldid=464531898 Contributors: -dennis-, 1ForTheMoney, A Man In Black, A. di M., ABF, Aardvark23, Abdullais4u, Adrideba, Aesir.le, Aknochel, Alansohn, Allstarecho, Altenmann, Alyjack, AnOddName, Anaxial, AndersFeder, Andrew c, AndrewN, Andrius.v, Angelo souti, AnonMoos, Anonymi, Antixt, Archelon, Art LaPella, Artur80, Asmeurer, Atomicthumbs, AxelBoldt, Baad, Bambaiah, Bbbl67, Bcody80, Bcorr, Ben MacDui, BenRG, Bender235, Benjiboy187, Benplowman, Betterusername, Bevo, Bhny, Big Brother 1984, Biker Biker, Bjankuloski06en, BobertWABC, Bodhitha, Bondegezou, Bookofjude, Boson15, Brainssturm, Brian Fenton, Brians, Britannica, Bryan Derksen, Bubba73, BullRangifer, Buster79, C S, CIS, CYD, Cadmasteradam, Caknuck, Calmer Waters, CamB424, CamB4242, CesarB, Cgd8d, Cgwaldman, CharlesC, Chetvorno, ChiZeroOne, Chreod, Christopher Thomas, Chuckupd, Cinkcool, ClaudeMuncey, Closedmouth, Comet Tuttle, Consumed Crustacean, Cructacean, D'Agosta, DBGustavson, DKqwerty, DMurphy, Daniel C, DannyDaWriter, Dante Alighieri, Dauto, Dave3457, David spector, Dbachmann, DeadlyMETAL, Deceglie, Dekker451, Dirac1933, Dirkbb, Discospinster, Diza, Donarreiskoffer, DrGaellon, Dragon of the Pants, Dratman, Drcooljoe, Drmies, DÅ‚ugosz, EchetusXe, Edderso, Edward, Ehn, Eikern, El C, ElfQrin, Eliga, Endersdouble, Epastore, Er ouz, Eritain, Ernsts, Ervin Goldfain, Escape Orbit, Excirial, Fatram, Fiziker, Fleisher, Fleminra, Flyguy649, Foobar, Foober, Foonle77, Fotoni, Frglee, Frymaster, Fæ, GDallimore, Gaurav, Giandrea, Giftlite, Gil987, Giuliopp, Gobbledygeek, Goethean, Golbez, Goudzovski, GregorB, Gurch, Gwib, Hadal, Hairy Dude, Harold f, Harp, Headbomb, Hellbus, Herbee, Herk1955, Heron, Higgshunter, Hippypink, I hate whitespace, Icairns, Iknowyourider, Ilmari Karonen, Impunv, Infestor, Irenan, Itinerant, Itinerant1, Iwpg, J M Rice, J mcandrews, J.delanoy, JCSantos, JTiago, JabberWok, Jacques Antoine, Jagged 85, Jason Quinn, JasonAQuest, Jc odcsmf, Jde123, Jdigitalbath, Jehochman, Jezzabr, Jfromcanada, Jgwacker, Jimtpat, Jkl, JohnArmagh, Johnflux, Jomoal99, JonathanDP81, Jonburchel, Jonbutterworth, Jor63, Joriki, JorisvS, Josh Cherry, Jpod2, Jtuggle, Justinrossetti, KHamsun, Kaihsu, KapilTagore, Kbdank71, Kbk, Kborland, Keith-264, Kencf0618, Kendrick7, Kenneth Dawson, Kgf0, Koavf, Kocio, Article Sources and Contributors 77

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Peter Higgs Source: http://en.wikipedia.org/w/index.php?oldid=464080346 Contributors: 28421u2232nfenfcenc, AVM, Abanima, Ahoerstemeier, Ancheta Wis, Anterior99, Badger Drink, Ben MacDui, Bender235, Benjaminevans82, Blu Matt, Bogdangiusca, Brad7777, Brighterorange, CJLL Wright, CRKingston, Charles Matthews, Coyets, Curps, D6, Dan berliner, DancingPhilosopher, David Gerard, David from Downunder, Dcoetzee, Dirac1933, Dottydotdot, Duendeverde, Emerson7, Ephraim33, Eric Kvaalen, Fanica1995, Flaming Ferrari, Fragglet, G716, Gareth Owen, Gilliam, Gronteam, Headbomb, Herbee, Hugo999, Ian Page, Ironboy11, Irsample, Isidore, JamesBWatson, Jamiemaloneyscoreg, Jandalhandler, Japanese Searobin, Jaraalbe, Jewce, KTC, Khukri, Laputian, Mary at CERN, Maybeheard21, Memming, Moose-32, OoberMick, Pamri, Pdfpdf, Phe, RS1900, Random18, Ratarsed, Rdblnwr, Richard L. Peterson, RodC, Rsabbatini, SCZenz, Salsb, Scythia, Simeon, SlimVirgin, Smallweed, Snowmanradio, Snufking, StewartMH, SylviaStanley, Taagane19, Tassedethe, The wub, Timrollpickering, Tpbradbury, Travelbybus, Urvabara, Vernon39, Woohookitty, XJamRastafire, YUL89YYZ, 61 anonymous edits

List of particles Source: http://en.wikipedia.org/w/index.php?oldid=463662603 Contributors: 041744, 112358sam, 49, A. di M., Adam Krellenstein, Adanadhel, Aegnor.erar, Ahoerstemeier, Alensha, Anaxial, Anonymous Dissident, Antixt, Arch dude, ArnoldReinhold, Asbestos, Atomic7732, AubreyEllenShomo, Audrius u, AxelBoldt, Axelfoley12, Axl, Azcolvin429, Bambaiah, Bazza 7, Bdesham, Betacommand, Bevo, Bissinger, Blennow, Bluetryst, Bodhitha, CBDunkerson, CalamusFortis, Cbuckley, Cedrus-Libani, Chaos, Charles Matthews, CityOfSilver, CommonsDelinker, Count Iblis, Cstmoore, D6, Dan Guan, Danny, DannyWilde, David Ko, Derdeib, Diffequa, Dirac1933, Discospinster, Docu, Donarreiskoffer, Dratman, Dstary, Duncan.france, Eddideigel, El C, Elmoosecapitan, EmilJ, Ems57fcva, Ernsts, Escalona, Evercat, Explicit, Favonian, Figureskatingfan, Flewis, Flickboy, FrozenMan, Gbrandt, Giftlite, Goudzovski, Graphite Elbow, GregorB, HESUPERMAN, Hamish a e fowler, Happy-melon, Hayne, Headbomb, Herbee, Hydrargyrum, Icairns, Icalanise, Inner Earth, Iridescent, Ishvara7, JLM, JPMasseo, JRSpriggs, JarlaxleArtemis, Jcimorra, Jeremy Henty, Jitse Niesen, Jmrowland, John, Johnman239, Jojhutton, Joshmt, Juancnuno, KDesk, Karam.Anthony.K, Khazar, Krash, L Kensington, LFaraone, Laurascudder, Lexicon, Linas, Lseixas, Mac Davis, Master of Puppets, Maurice Carbonaro, McGeddon, Merovingian, Minghong, Mysidia, Nikai, Nolimitownass, OlEnglish, Oleg Alexandrov, OliverHarris, OllieFury, Omicronpersei8, Paolo.dL, Pengo, Physicist, Physicistjedi, Poulpy, Quadell, Quibik, Quilbert, Qutezuce, Qwyrxian, RJFJR, RSStockdale, Radical Mallard, Raistuumum, RandorXeus, Raul654, RetiredUser2, Rjwilmsi, Rmhermen, Rotiro, SCZenz, Sadisticsuburbanite, Salsa Shark, Salsb, Sanders muc, Schneelocke, Scorpion0422, Scottmsg, SkyLined, Slakr, Stan Shebs, Stevertigo, StewartMH, Strait, Suslindisambiguator, Susvolans, TUSHANT JHA, TauLibrus, Tavilis, Teles, TenOfAllTrades, TheEditrix2, TimothyRias, Tom Lougheed, Tomvds, TriTertButoxy, TwistOfCain, Twocount, Tyco.skinner, Urvabara, Van helsing, VovanA, Vulcan Hephaestus, Vuvar1, Waperkins, WereSpielChequers, WookieInHeat, Xerxes314, Zojj, 272 anonymous edits Image Sources, Licenses and Contributors 79 Image Sources, Licenses and Contributors

Image:First Gold Beam-Beam Collision Events at RHIC at 100 100 GeV c per beam recorded by STAR.jpg Source: http://en.wikipedia.org/w/index.php?title=File:First_Gold_Beam-Beam_Collision_Events_at_RHIC_at_100_100_GeV_c_per_beam_recorded_by_STAR.jpg License: Creative Commons ,ﻣﺎﻧﻔﯽ ,Attribution-Sharealike 2.0 Contributors: Dbc334, Dmgultekin, Doodledoo, FlickreviewR, HAH, Herald Alberich, Kuaile Long, Odie5533, Romanm, Roomba, Saperaud, Túrelio, Yarnalgo 5 anonymous edits Image:Standard Model of Elementary Particles.svg Source: http://en.wikipedia.org/w/index.php?title=File:Standard_Model_of_Elementary_Particles.svg License: Creative Commons Attribution 3.0 Contributors: MissMJ Image:Elementary particle interactions.svg Source: http://en.wikipedia.org/w/index.php?title=File:Elementary_particle_interactions.svg License: Public domain Contributors: en:User:TriTertButoxy, User:Stannered Image:Particle chart Log.svg Source: http://en.wikipedia.org/w/index.php?title=File:Particle_chart_Log.svg License: Public Domain Contributors: Arivero, 1 anonymous edits Image:Hqmc-vector.svg Source: http://en.wikipedia.org/w/index.php?title=File:Hqmc-vector.svg License: Creative Commons Attribution 3.0 Contributors: VermillionBird Image:Particle overview.svg Source: http://en.wikipedia.org/w/index.php?title=File:Particle_overview.svg License: Creative Commons Attribution-Sharealike 3.0 Contributors: Headbomb File:CMS Higgs-event.jpg Source: http://en.wikipedia.org/w/index.php?title=File:CMS_Higgs-event.jpg License: unknown Contributors: Lucas Taylor File:AIP-Sakurai-best.JPG Source: http://en.wikipedia.org/w/index.php?title=File:AIP-Sakurai-best.JPG License: Public Domain Contributors: self File:Higgs, Peter (1929).jpg Source: http://en.wikipedia.org/w/index.php?title=File:Higgs,_Peter_(1929).jpg License: Creative Commons Attribution-Sharealike 2.0 Contributors: Gert-Martin Greuel File:One-loop-diagram.svg Source: http://en.wikipedia.org/w/index.php?title=File:One-loop-diagram.svg License: GNU Free Documentation License Contributors: JabberWok Image:Higgs-Boson-March-2011.png Source: http://en.wikipedia.org/w/index.php?title=File:Higgs-Boson-March-2011.png License: Creative Commons Zero Contributors: aesir.le Image:Gluon-top-higgs.svg Source: http://en.wikipedia.org/w/index.php?title=File:Gluon-top-higgs.svg License: GNU Free Documentation License Contributors: http://en.wikipedia.org/wiki/User:JabberWok Image:BosonFusion-Higgs.svg Source: http://en.wikipedia.org/w/index.php?title=File:BosonFusion-Higgs.svg License: GNU Free Documentation License Contributors: derivative work: ~ Booya Bazooka BosonFusion-Higgs.png: User:Harp 12:43, 28 March 2007 Image:Mecanismo de Higgs PH.png Source: http://en.wikipedia.org/w/index.php?title=File:Mecanismo_de_Higgs_PH.png License: GNU Free Documentation License Contributors: Yrithinnd Image:LHC.svg Source: http://en.wikipedia.org/w/index.php?title=File:LHC.svg License: Creative Commons Attribution-Sharealike 2.5 Contributors: User:Harp File:BosonFusion-Higgs.svg Source: http://en.wikipedia.org/w/index.php?title=File:BosonFusion-Higgs.svg License: GNU Free Documentation License Contributors: derivative work: ~ Booya Bazooka BosonFusion-Higgs.png: User:Harp 12:43, 28 March 2007 File:Location Large Hadron Collider.PNG Source: http://en.wikipedia.org/w/index.php?title=File:Location_Large_Hadron_Collider.PNG License: Creative Commons Attribution-Sharealike 2.0 Contributors: diverse contributors; mashup by User:Zykure File:LHC quadrupole magnets.jpg Source: http://en.wikipedia.org/w/index.php?title=File:LHC_quadrupole_magnets.jpg License: Creative Commons Attribution 2.0 Contributors: Andrius.v File:Construction of LHC at CERN.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Construction_of_LHC_at_CERN.jpg License: GNU Free Documentation License Contributors: Andrius.v, Deadstar, Square87 File:Higgs, Peter (1929)3.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Higgs,_Peter_(1929)3.jpg License: Creative Commons Attribution-Sharealike 2.0 Contributors: Gert-Martin Greuel Image:Baryon decuplet.svg Source: http://en.wikipedia.org/w/index.php?title=File:Baryon_decuplet.svg License: Public Domain Contributors: Original uploader was Wierdw123 at en.wikipedia (Original text : Wierdw123 (talk)) Image:Quark structure proton.svg Source: http://en.wikipedia.org/w/index.php?title=File:Quark_structure_proton.svg License: Creative Commons Attribution-Sharealike 2.5 Contributors: Made by Arpad Horvath Image:Noneto mesônico de spin 0.png Source: http://en.wikipedia.org/w/index.php?title=File:Noneto_mesônico_de_spin_0.png License: Public Domain Contributors: E2m, Pieter Kuiper, Stannered Image:Helium atom QM.svg Source: http://en.wikipedia.org/w/index.php?title=File:Helium_atom_QM.svg License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: User:Yzmo License 80 License

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