DOCSLIB.ORG

1 Quark Model of Hadrons

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
1 Quark Model of Hadrons 1 QUARK MODEL OF HADRONS 1 Quark model of hadrons 1.1 Symmetries and evidence for quarks 1 Hadrons as bound states of quarks – point-like spin- 2 objects, charged (‘coloured’) under the strong force. Baryons as qqq combinations. Mesons as qq¯ combinations. 1 Baryon number = 3 (N(q) − N(¯q)) and its conservation. You are not expected to know all of the names of the particles in the baryon and meson multiplets, but you are expected to know that such multiplets exist, and to be able to interpret them if presented with them. You should also know the quark contents of the simple light baryons and mesons (and their anti-particles): p = (uud) n = (udd) π0 = (mixture of uu¯ and dd¯) π+ = (ud¯) K0 = (ds¯) K+ = (us¯). and be able to work out others with some hints. P 1 + Lowest-lying baryons as L = 0 and J = 2 states of qqq. P 3 + Excited versions have L = 0 and J = 2 . Pauli exclusion and non existence of uuu, ddd, sss states in lowest lying multiplet. Lowest-lying mesons as qq¯0 states with L = 0 and J P = 0−. First excited levels (particles) with same quark content have L = 0 and J P = 1−. Ability to explain contents of these multiplets in terms of quarks. J/Ψ as a bound state of cc¯ Υ (upsilon) as a bound state of b¯b. Realization that these are hydrogenic-like states with suitable reduced mass (c.f. positronium), and subject to the strong force, so with energy levels paramaterized by αs rather than αEM Top is very heavy and decays before it can form hadrons, so no top hadrons exist. You should (for example) be able to ⇒ draw quark-flow diagrams for production and decay of resonances; ⇒ determine if a reaction can proceed via the strong force (no flavour changes, only strongly-interacting particles participate); ⇒ be able to apply Breit-Wigner formula to extract/interpret properties of new resonances (N.B. CM energy needed – first year relativity). Isospin was touched on in the problem set but is non-examinable. 1 1.2 Nucleons as bound states of quarks 2 THE STANDARD MODEL 1.2 Nucleons as bound states of quarks p = uud, n = udd You should be able to draw quark-flow and Feynman diagrams at the quark level for reactions involving protons and neutrons, e.g. β± decay, electron capture, solar reactions 1.3 Deep inelastic scattering Scattering of a very high-momentum ( 1 GeV) lepton probe from some target hadron (usually p or n). Probe has small wavelength and so scatters from what appear to be “temporarily free quarks” inside the hadron, rather than from hadron as a whole. Subsequently outgoing quarks cannot exist as free objects so generate ‘jets’ of hadrons along their direction of motion. Probe may be neutrino or charged lepton (usually e±). You should be able to draw Feynman diagrams at the quark level for charged- current (W ±) and neutral-current (Z0 and/or γ as appropriate). Vertex factors and relativistic propagators 1/ P · P − m2 for virtual particles You should be able to ⇒ work out the energy-momentum of the ZMF, and of the virtual/intermediate boson ⇒ indicate the vertex couplings and calculate the propagator factor(s). (Note, need basic relativistic kinematics again here. Life is easier with 4-vectors.) 2 The Standard Model 2.1 Quark and lepton families Members of the families (a.k.a. generations): quarks: (u, d), (c, s), (t, b); charged 1 leptons: e, µ, τ and their associated neutrinos. Spin= 2 . Point-like nature. Some idea of mass. Quantum numbers (baryon number, lepton number, strangeness, charm, bottomness, electric charge, colour charge [non-examinable: isospin]). Fundamental interactions: ⇒ electromagnetic charges and couplings of each to photon (vertex factor of Qge where Q is the charge in units of proton charge); ⇒ strong charges and couplings of each quark to the gluon (vertex factor of gs for quarks only) ; 2 2.2 Strong interaction, confinement 2 THE STANDARD MODEL ± 0 ⇒ vertices involving W and Z with vertex factors ≈ gw (W couplings involving quarks of different generations are in fact suppressed). Feynman diagrams involving each vertex. Flavour mixing: flavour changes only occur in quarks via W ± boson (weak eigen- states 6= mass/energy eigenstates). Inter-family mixing (e.g. usW or bcW ver- tices) suppressed relative to mixing within families (e.g. udW or csW vertices). No flavour-changing neutral currents. Flavour mixing: flavour changes only occur in leptons via neutrino oscillations (mass/energy eigenstates 6= flavour/weak eigenstates). 2.2 Strong interaction, confinement Force transmitted by massless, coloured gluons. Force is strong since αs = gs/(4π) ≈ 1. Gluons carry colour ⇒ gluon self-interactions. As quarks pulled apart potential energy continues to increase. If quarks pulled far apart, the energy in the field is transformed into new particles. qq¯ pairs are pulled out of the vacuum, allowing the formation of multiple colour-neutral states (mesons and baryons). Result is jets of hadrons in direction of original quark and/or anti- quark momenta. (The sprays or jets are created because of the relativistic headlight effect). Net result: quarks can’t be found free in nature – they are confined within colour- neutral hadrons of size ∼ fm. 2.3 Weak interaction Mediated by W ± and Z0 bosons. Couple to quarks and leptons with vertex factors ∼ gw. Universality of lepton couplings. For quarks W couplings suppressed for inter-generational W qq0 vertices. Weak boson vertices violate parity: amplitudes not the same after the parity oper- ation P: x → −x. Helicity is projection of spin in momentum direction h = S·p/|p|. We showed in the examples that the helicity eigen-states change sign under a parity operation. W ± bosons couple preferentially to negative-helicity quarks and leptons, and to positive- helicity anti-quarks and anti-leptons. In the relativistic limit the W ± bosons couple only to those helicities.1 The reactions rates are very, very nearly the same after the combined operations of parity (x → −x) and charge conjugation (replacement of particles with their corresponding anti-particles). The combined operation is known as CP. 1Non-examinable: in the relativistic limit the helicity of a spin-half particle coincides with a property of a spin-half particle which is called its chirality or ‘handedness’. 3 2.4 Production and decay of the W and Z bosons2 THE STANDARD MODEL 2.4 Production and decay of the W and Z bosons Production in hadron and lepton collisions. (Relativistic) calculations involving energy, momentum, CM energy. Experimental identification of final states involving leptons, jets and missing energy & momentum from neutrinos. Width of the Z0: + − + − + − Γ(Z) = Γ(qq,¯ q ∈ u, d, s, c, b) + Γe e + Γµ µ + Γµ µ + Nν × Γ(νν¯). Can measure full width from width of Breit-Wigner. Can calculate the partial widths to the visible final states from their rates. With a calculation of Γ(νν¯) we can then 0 infer Nν , the number of light neutrinos that couple to the Z . Find Nν = 3 to high precision. Good evidence for only three generations of leptons (and so we presume quarks). 2.5 Neutrino oscillations Individual lepton flavour [for any individual ` ∈ {e, µ , τ}, the lepton flavour number − + ± is L` = N(` ) − N(` ) + N(ν`) − N(¯ν`)] conserved at W vertices, however over long distances neutrinos found to change flavour. Experiments consistent with quantum oscillations. Calculation for two-state system (handout). Results in lepton flavour violation (but not violation of total lepton number: L = Le + Lµ + Lτ . Examples of solar and atmospheric neutrino oscillations. Implication is that neutrinos must have (small, sub-eV) mass. 4 3 EXAMPLE: B1 2006 QUESTION 6 3 Example: B1 2006 question 6 Link to 2006 B1 paper The following list gives some combinations of quarks. State which combinations occur in nature, and in each such case give an example. q, qq, qqq, qqqq, qq, qqq, qqqq, qqqqq, qqq, qqq The D+ meson is the lowest-mass charmed meson with a mass of 1869 MeV/c2. The (D+)∗ meson has a mass of 2010 MeV/c2 and has spin-parity J P = 1−. Give the spin and parity of the D+ and explain how the D+ and (D+)∗ mesons fit into the quark model. + ∗0 + The D may decay into K µ νµ. What is the maximum possible momentum the muon could have in a decay in which the D+ has total energy of 1885 MeV? Enumerate the likely decay modes of the (D+)∗ meson and draw a Feynman diagram or quark flow diagram for each. ∗0 [The mass of K is 892 MeV/c2.] Of the combinations given, the candidates that might exist in nature must be colourless. The quark–antiquark combinations that can be made colourless are: quarks example quarks example hadron qqq uud proton qq¯ ud¯ π+ meson q¯q¯q¯ u¯d¯d¯ anti-neutron qqqqq¯ – – Only the baryons (qqq), mesons (qq¯) and the anti-baryons (q¯q¯q¯), are observed in nature. (The baryon–meson ‘molecule’ qqqqq¯ has not been confirmed to exist.) The D+ is the lowest lying charmed meson. It must therefore be a ground-state combination of a charmed quark with a first-generation quark. To get the positive charge the charm must be in the form of c rather than c¯ and the anti-quark must be a d¯; so it has quark content cd¯. All the lowest-lying mesons for light quark content have J = S = L = 0 and negative parity, so we would expect the same to be true for the D+. J P = 0− The (D+)∗ is an excited state of the D+.
Load more

Recommended publications
  • An Introduction to the Quark Model
    An Introduction to the Quark Model Garth Huber Prairie Universities Physics Seminar Series, November, 2009. Particles in Atomic Physics • View of the particle world as of early 20th Century. • Particles found in atoms: –Electron – Nucleons: •Proton(nucleus of hydrogen) •Neutron(e.g. nucleus of helium – α-particle - has two protons and two neutrons) • Related particle mediating electromagnetic interactions between electrons and protons: Particle Electric charge Mass – Photon (light!) (x 1.6 10-19 C) (GeV=x 1.86 10-27 kg) e −1 0.0005 p +1 0.938 n 0 0.940 γ 0 0 Dr. Garth Huber, Dept. of Physics, Univ. of Regina, Regina, SK S4S0A2, Canada. 2 Early Evidence for Nucleon Internal Structure • Apply the Correspondence Principle to the Classical relation for q magnetic moment: µ = L 2m • Obtain for a point-like spin-½ particle of mass mp: qqeq⎛⎞⎛⎞ µ == =µ ⎜⎟ ⎜⎟ N 2222memepp⎝⎠ ⎝⎠ 2 Experimental values: µp=2.79 µN (p) µn= -1.91 µN (n) • Experimental values inconsistent with point-like assumption. • In particular, the neutron’s magnetic moment does not vanish, as expected for a point-like electrically neutral particle. This is unequivocal evidence that the neutron (and proton) has an internal structure involving a distribution of charges. Dr. Garth Huber, Dept. of Physics, Univ. of Regina, Regina, SK S4S0A2, Canada. 3 The Particle Zoo • Circa 1950, the first particle accelerators began to uncover many new particles. • Most of these particles are unstable and decay very quickly, and hence had not been seen in cosmic ray experiments. • Could all these particles be fundamental? Dr. Garth Huber, Dept.
    [Show full text]
  • The Structure of Quarks and Leptons
    The Structure of Quarks and Leptons They have been , considered the elementary particles ofmatter, but instead they may consist of still smaller entities confjned within a volume less than a thousandth the size of a proton by Haim Harari n the past 100 years the search for the the quark model that brought relief. In imagination: they suggest a way of I ultimate constituents of matter has the initial formulation of the model all building a complex world out of a few penetrated four layers of structure. hadrons could be explained as combina­ simple parts. All matter has been shown to consist of tions of just three kinds of quarks. atoms. The atom itself has been found Now it is the quarks and leptons Any theory of the elementary particles to have a dense nucleus surrounded by a themselves whose proliferation is begin­ fl. of matter must also take into ac­ cloud of electrons. The nucleus in turn ning to stir interest in the possibility of a count the forces that act between them has been broken down into its compo­ simpler-scheme. Whereas the original and the laws of nature that govern the nent protons and neutrons. More recent­ model had three quarks, there are now forces. Little would be gained in simpli­ ly it has become apparent that the pro­ thought to be at least 18, as well as six fying the spectrum of particles if the ton and the neutron are also composite leptons and a dozen other particles that number of forces and laws were thereby particles; they are made up of the small­ act as carriers of forces.
    [Show full text]
  • Properties of Baryons in the Chiral Quark Model
    Properties of Baryons in the Chiral Quark Model Tommy Ohlsson Teknologie licentiatavhandling Kungliga Tekniska Hogskolan¨ Stockholm 1997 Properties of Baryons in the Chiral Quark Model Tommy Ohlsson Licentiate Dissertation Theoretical Physics Department of Physics Royal Institute of Technology Stockholm, Sweden 1997 Typeset in LATEX Akademisk avhandling f¨or teknologie licentiatexamen (TeknL) inom ¨amnesomr˚adet teoretisk fysik. Scientific thesis for the degree of Licentiate of Engineering (Lic Eng) in the subject area of Theoretical Physics. TRITA-FYS-8026 ISSN 0280-316X ISRN KTH/FYS/TEO/R--97/9--SE ISBN 91-7170-211-3 c Tommy Ohlsson 1997 Printed in Sweden by KTH H¨ogskoletryckeriet, Stockholm 1997 Properties of Baryons in the Chiral Quark Model Tommy Ohlsson Teoretisk fysik, Institutionen f¨or fysik, Kungliga Tekniska H¨ogskolan SE-100 44 Stockholm SWEDEN E-mail: [email protected] Abstract In this thesis, several properties of baryons are studied using the chiral quark model. The chiral quark model is a theory which can be used to describe low energy phenomena of baryons. In Paper 1, the chiral quark model is studied using wave functions with configuration mixing. This study is motivated by the fact that the chiral quark model cannot otherwise break the Coleman–Glashow sum-rule for the magnetic moments of the octet baryons, which is experimentally broken by about ten standard deviations. Configuration mixing with quark-diquark components is also able to reproduce the octet baryon magnetic moments very accurately. In Paper 2, the chiral quark model is used to calculate the decuplet baryon ++ magnetic moments. The values for the magnetic moments of the ∆ and Ω− are in good agreement with the experimental results.
    [Show full text]
  • Supersymmetry in the Quark-Diquark and the Baryon-Meson Systems
    Acta Polytechnica Vol. 51 No. 1/2011 Supersymmetry in the Quark-Diquark and the Baryon-Meson Systems S. Catto, Y. Choun Abstract A superalgebra extracted from the Jordan algebra of the 27 and 27¯ dim. representations of the group E6 is shown to be relevant to the description of the quark-antidiquark system. A bilocal baryon-meson field is constructed from two quark-antiquark fields. In the local approximation the hadron field is shown to exhibit supersymmetry which is then extended to hadronic mother trajectories and inclusion of multiquark states. Solving the spin-free Hamiltonian with light quark masses we develop a new kind of special function theory generalizing all existing mathematical theories of confluent hypergeometric type. The solution produces extra “hidden” quantum numbers relevant for description of supersymmetry and for generating new mass formulas. Keywords: supersymmetry, relativistic quark model. A colored supersymmetry scheme based on SU(3)c × SU(6/1) Algebraic realization of supersymmetry and its experimental consequences based on supergroups of type SU(6/21) and color algebras based on split octonionic units ui (i =0,...,3) was considered in our earlier papers[1,2,3,4,5].Here,weaddtothemthelocalcolorgroupSU(3)c. We could go to a smaller supergroup having SU(6) as a subgroup. With the addition of color, such a supergroup is SU(3)c × SU(6/1). The fun- damental representation of SU(6/1) is 7-dimensional which decomposes into a sextet and a singlet under the spin-flavor group. There is also a 28-dimensional representation of SU(7). Under the SU(6) subgroup it has the decomposition 28 = 21 + 6 + 1 (1) Hence, this supermultiplet can accommodate the bosonic antidiquark and fermionic quark in it, provided we are willing to add another scalar.
    [Show full text]
  • The Quark Model for Leptons a Theoretical Basis for a Hadronic Structure of Leptons According to Quantum Chromodynamics Criteria
    DOI: 10.36178/inv.phys.1103020 “INVESTIGATIONS” in Physics The Quark Model for Leptons A Theoretical Basis for a Hadronic Structure of Leptons According to Quantum Chromodynamics Criteria Mauro Santosuosso∗ Study Center for the Physical Investigation of Reality, V.le F. Cecconi, 17 - 00015 Monterotondo (RM), Italy April 3, 2021 Abstract Unlike preon models, in which it is hypothesized that the existence of con- stituents is more elementary than those we already know, a substantially different model is proposed here: The proposed model does not foresee new particles, but instead, focuses on a rather unconventional combination of the well-known quarks inside leptons. The idea arises from the observation of some “vacancies” in the baryon octet: All the triplets of quarks of the same flavor with total angular mo- mentum J P = 1/2+ are denied by the Pauli exclusion principle. However, bypassing this prohibition because of the real possibility of introducing a form of “conditioned” symmetry, we arrive at a result – for down-type quark triplets only – in which two of the three quarks are strongly joined together in a di-quark, a two-color boson with an integer spin, which binds very closely to the third quark, together forming a charged colorless lepton. The neutral partners are given by the combination of the di-quark with the up-type quark, and with the related charged lepton, they form an isospin doublet. Thus, the di-quark boson becomes the carrier of the lepton number, which discriminates the three families. Studying the color forces according to the principles of quantum chromodynamics (QCD) shows how the quark – di-quark cou- pling is considerably higher than the quark – quark or quark – antiquark coupling, explaining why leptons today seem to be devoid of structure and unexpectedly justifying the smallness of their masses compared with those of the corresponding hadrons.
    [Show full text]
  • Sakata Model Precursor 2: Eightfold Way, Discovery of Ω- Quark Model: First Three Quarks A
    Introduction to Elementary Particle Physics. Note 20 Page 1 of 17 THREE QUARKS: u, d, s Precursor 1: Sakata Model Precursor 2: Eightfold Way, Discovery of ΩΩΩ- Quark Model: first three quarks and three colors Search for free quarks Static evidence for quarks: baryon magnetic moments Early dynamic evidence: - πππN and pN cross sections - R= σσσee →→→ hadrons / σσσee →→→ µµµµµµ - Deep Inelastic Scattering (DIS) and partons - Jets Introduction to Elementary Particle Physics. Note 20 Page 2 of 17 Sakata Model 1956 Sakata extended the Fermi-Yang idea of treating pions as nucleon-antinucleon bound states, e.g. π+ = (p n) All mesons, baryons and their resonances are made of p, n, Λ and their antiparticles: Mesons (B=0): Note that there are three diagonal states, pp, nn, ΛΛ. p n Λ Therefore, there should be 3 independent states, three neutral mesons: π0 = ( pp - nn ) / √2 with isospin I=1 - - p ? π K X0 = ( pp + nn ) / √2 with isospin I=0 0 ΛΛ n π+ ? K0 Y = with isospin I=0 Or the last two can be mixed again… + 0 Λ K K ? (Actually, later discovered η and η' resonances could be interpreted as such mixtures.) Baryons (B=1): S=-1 Σ+ = ( Λ p n) Σ0 = ( Λ n n) mixed with ( Λ p p) what is the orthogonal mixture? Σ- = ( Λ n p) S=-2 Ξ- = ( Λ Λp) Ξ- = ( Λ Λn) S=-3 NOT possible Resonances (B=1): ∆++ = (p p n) ∆+ = (p n n) mixed with (p p p) what is the orthogonal mixture? ∆0 = (n n n) mixed with (n p p) what is the orthogonal mixture? ∆- = (n n p) Sakata Model was the first attempt to come up with some plausible internal structure that would allow systemizing the emerging zoo of hadrons.
    [Show full text]
  • Mass Generation Via the Higgs Boson and the Quark Condensate of the QCD Vacuum
    Pramana – J. Phys. (2016) 87: 44 c Indian Academy of Sciences DOI 10.1007/s12043-016-1256-0 Mass generation via the Higgs boson and the quark condensate of the QCD vacuum MARTIN SCHUMACHER II. Physikalisches Institut der Universität Göttingen, Friedrich-Hund-Platz 1, D-37077 Göttingen, Germany E-mail: [email protected] Published online 24 August 2016 Abstract. The Higgs boson, recently discovered with a mass of 125.7 GeV is known to mediate the masses of elementary particles, but only 2% of the mass of the nucleon. Extending a previous investigation (Schumacher, Ann. Phys. (Berlin) 526, 215 (2014)) and including the strange-quark sector, hadron masses are derived from the quark condensate of the QCD vacuum and from the effects of the Higgs boson. These calculations include the π meson, the nucleon and the scalar mesons σ(600), κ(800), a0(980), f0(980) and f0(1370). The predicted second σ meson, σ (1344) =|ss¯, is investigated and identified with the f0(1370) meson. An outlook is given on the hyperons , 0,± and 0,−. Keywords. Higgs boson; sigma meson; mass generation; quark condensate. PACS Nos 12.15.y; 12.38.Lg; 13.60.Fz; 14.20.Jn 1. Introduction adds a small additional part to the total constituent- quark mass leading to mu = 331 MeV and md = In the Standard Model, the masses of elementary parti- 335 MeV for the up- and down-quark, respectively [9]. cles arise from the Higgs field acting on the originally These constituent quarks are the building blocks of the massless particles. When applied to the visible matter nucleon in a similar way as the nucleons are in the case of the Universe, this explanation remains unsatisfac- of nuclei.
    [Show full text]
  • Phenomenological Review on Quark–Gluon Plasma: Concepts Vs
    Review Phenomenological Review on Quark–Gluon Plasma: Concepts vs. Observations Roman Pasechnik 1,* and Michal Šumbera 2 1 Department of Astronomy and Theoretical Physics, Lund University, SE-223 62 Lund, Sweden 2 Nuclear Physics Institute ASCR 250 68 Rez/Prague,ˇ Czech Republic; [email protected] * Correspondence: [email protected] Abstract: In this review, we present an up-to-date phenomenological summary of research developments in the physics of the Quark–Gluon Plasma (QGP). A short historical perspective and theoretical motivation for this rapidly developing field of contemporary particle physics is provided. In addition, we introduce and discuss the role of the quantum chromodynamics (QCD) ground state, non-perturbative and lattice QCD results on the QGP properties, as well as the transport models used to make a connection between theory and experiment. The experimental part presents the selected results on bulk observables, hard and penetrating probes obtained in the ultra-relativistic heavy-ion experiments carried out at the Brookhaven National Laboratory Relativistic Heavy Ion Collider (BNL RHIC) and CERN Super Proton Synchrotron (SPS) and Large Hadron Collider (LHC) accelerators. We also give a brief overview of new developments related to the ongoing searches of the QCD critical point and to the collectivity in small (p + p and p + A) systems. Keywords: extreme states of matter; heavy ion collisions; QCD critical point; quark–gluon plasma; saturation phenomena; QCD vacuum PACS: 25.75.-q, 12.38.Mh, 25.75.Nq, 21.65.Qr 1. Introduction Quark–gluon plasma (QGP) is a new state of nuclear matter existing at extremely high temperatures and densities when composite states called hadrons (protons, neutrons, pions, etc.) lose their identity and dissolve into a soup of their constituents—quarks and gluons.
    [Show full text]
  • 15. Quark Model 1 15
    15. Quark model 1 15. Quark Model Revised August 2017 by C. Amsler (Stefan Meyer Institute for Subatomic Physics, Vienna), T. DeGrand (University of Colorado, Boulder), and B. Krusche (University of Basel). 15.1. Quantum numbers of the quarks Quantum chromodynamics (QCD) is the theory of the strong interactions. QCD is a quantum field theory and its constituents are a set of fermions, the quarks, and gauge bosons, the gluons. Strongly interacting particles, the hadrons, are bound states of quark and gluon fields. As gluons carry no intrinsic quantum numbers beyond color charge, and because color is believed to be permanently confined, most of the quantum numbers of strongly interacting particles are given by the quantum numbers of their constituent quarks and antiquarks. The description of hadronic properties which strongly emphasizes the role of the minimum-quark-content part of the wave function of a hadron is generically called the quark model. It exists on many levels: from the simple, almost dynamics-free picture of strongly interacting particles as bound states of quarks and antiquarks, to more detailed descriptions of dynamics, either through models or directly from QCD itself. The different sections of this review survey the many approaches to the spectroscopy of strongly interacting particles which fall under the umbrella of the quark model. Table 15.1: Additive quantum numbers of the quarks. d u s c b t Q – electric charge 1 + 2 1 + 2 1 + 2 − 3 3 − 3 3 − 3 3 I – isospin 1 1 0 0 0 0 2 2 1 1 Iz – isospin z-component + 0 0 0 0 − 2 2 S – strangeness 0 0 1 0 0 0 − C – charm 0 0 0 +1 0 0 B – bottomness 0 0 0 0 1 0 − T – topness 0 0 0 0 0 +1 Quarks are strongly interacting fermions with spin 1/2 and, by convention, positive parity.
    [Show full text]
  • Quark-Gluon Plasma and the Early Universe
    Chapter 2 Quark-Gluon Plasma and the Early Universe There is now considerable evidence that the universe began as a fireball, the so called “Big-Bang”, with extremely high temperature and high energy density. At early enough times, the temperature was certainly high enough (T > 100 GeV) that all the known particles (including quarks, leptons, gluons, photons, Higgs bosons, W and Z) were extremely relativistic. Even the “strongly inter- acting” particles, quarks and gluons, would interact fairly weakly due to asymptotic freedom and perturbation theory should be sufficient to describe them. Thus this was a system of hot, weakly interacting color-charged particles, a quark-gluon plasma (QGP), in equilibrium with the other species. Due to asymptotic freedom, at sufficiently high temperature the quark-gluon plasma can be well-described using statistical mechanics as a free relativistic parton gas. In this Chapter, we explore the physics of QGP, perhaps the simplest system of strong-interaction particles that exists in the context of QCD. As the universe cooled during the subsequent expansion phase, the quarks, antiquarks, and gluons combined to form hadrons resulting in the baryonic matter that we observe today. The transition from quarks and gluons to baryons is a fascinating subject that has been difficult to address quantitatively. However, we will discuss this transition by considering the basic physics issues without treating the quantitative details. At present there is a substantial effort in theoretical physics to address this transition by using high-level computational methods known as lattice gauge theory. This subject is somewhat technical and we will discuss it only very briefly.
    [Show full text]
  • How Quarks Became Real the First Dalitz Plot the First Dalitz Plot
    How Quarks became Real The first Dalitz Plot The first Dalitz Plot 1954 tau-theta analysis 1955: tau decay Dalitz plots in Cloud chamber………and emulsion 1960 A “new” application of Dalitz plots Strong interaction resonances = discovery of Sigma*(1385) Partner of Delta(1230) Beginning of the Decuplet…. Soon followed by Csi*(1530)….. And MGM Eightfold Way … and so Gell Mann invented the Eightfold Way with octets and decuplets and first spoke about it in 1961 at TIFR summer school in Bangalore …. with Dick Dalitz in the audience …. as recalled in 30 May email by G Rajasekran Email from G Rajasekran on origin of quark model: physics/0602131 During one of the lectures, Dalitz questioned him about the triplets. Why is he ignoring them? Email from G Rajasekran on origin of quark model: physics/0602131 During one of the lectures, Dalitz questioned him about the triplets. Why is he ignoring them? Gell-Mann managed to evade it, inspite of Dalitz's repeated questioning. If Gell-Mann had answered the question directly, quarks would have been born in Bangalore in 1961 instead of having to wait for another three years...." aaaaaaaaaaaaaaaaaaa O O L=1 What Dalitz had proposed O O O L=2 O PDG 2006 !!! 1650 1700 1700 1620 1675 1535 O 1520 O L=1 O 1520 1405 O O L=2 O 1910 1920 1905 1950 1720 1680 1977 : L=3 levels identified By 1974+ charm had established quarks as real even for sceptics But there are some quirks of quarks prior to that “November revolution” that Dalitz stimulated.
    [Show full text]
  • ELEMENTARY PARTICLES in PHYSICS 1 Elementary Particles in Physics S
    ELEMENTARY PARTICLES IN PHYSICS 1 Elementary Particles in Physics S. Gasiorowicz and P. Langacker Elementary-particle physics deals with the fundamental constituents of mat- ter and their interactions. In the past several decades an enormous amount of experimental information has been accumulated, and many patterns and sys- tematic features have been observed. Highly successful mathematical theories of the electromagnetic, weak, and strong interactions have been devised and tested. These theories, which are collectively known as the standard model, are almost certainly the correct description of Nature, to first approximation, down to a distance scale 1/1000th the size of the atomic nucleus. There are also spec- ulative but encouraging developments in the attempt to unify these interactions into a simple underlying framework, and even to incorporate quantum gravity in a parameter-free “theory of everything.” In this article we shall attempt to highlight the ways in which information has been organized, and to sketch the outlines of the standard model and its possible extensions. Classification of Particles The particles that have been identified in high-energy experiments fall into dis- tinct classes. There are the leptons (see Electron, Leptons, Neutrino, Muonium), 1 all of which have spin 2 . They may be charged or neutral. The charged lep- tons have electromagnetic as well as weak interactions; the neutral ones only interact weakly. There are three well-defined lepton pairs, the electron (e−) and − the electron neutrino (νe), the muon (µ ) and the muon neutrino (νµ), and the (much heavier) charged lepton, the tau (τ), and its tau neutrino (ντ ). These particles all have antiparticles, in accordance with the predictions of relativistic quantum mechanics (see CPT Theorem).
    [Show full text]