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The Standard Model and Beyond Maxim Perelstein, LEPP/Cornell U

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The Standard Model and Beyond Maxim Perelstein, LEPP/Cornell U The Standard Model and Beyond Maxim Perelstein, LEPP/Cornell U. NYSS APS/AAPT Conference, April 19, 2008 The basic question of particle physics: What is the world made of? What is the smallest indivisible building block of matter? Is there such a thing? In the 20th century, we made tremendous progress in observing smaller and smaller objects Today’s accelerators allow us to study matter on length scales as short as 10^(-18) m The world’s largest particle accelerator/collider: the Tevatron (located at Fermilab in suburban Chicago) 4 miles long, accelerates protons and antiprotons to 99.9999% of speed of light and collides them head-on, 2 The CDF million collisions/sec. detector The control room Particle Collider is a Giant Microscope! • Optics: diffraction limit, ∆min ≈ λ • Quantum mechanics: particles waves, λ ≈ h¯/p • Higher energies shorter distances: ∆ ∼ 10−13 cm M c2 ∼ 1 GeV • Nucleus: proton mass p • Colliders today: E ∼ 100 GeV ∆ ∼ 10−16 cm • Colliders in near future: E ∼ 1000 GeV ∼ 1 TeV ∆ ∼ 10−17 cm Particle Colliders Can Create New Particles! • All naturally occuring matter consists of particles of just a few types: protons, neutrons, electrons, photons, neutrinos • Most other known particles are highly unstable (lifetimes << 1 sec) do not occur naturally In Special Relativity, energy and momentum are conserved, • 2 but mass is not: energy-mass transfer is possible! E = mc • So, a collision of 2 protons moving relativistically can result in production of particles that are much heavier than the protons, “made out of” their kinetic energy • This is how most elementary particles are discovered! Another basic question: how did the Universe begin? High-energy particle collisions, today seen only in accelerators, were quite common in the high- temperature, high-density universe within the first second after the Big Bang! All our knowledge about subatomic physics is summarized in the Standard Model - the most successful Physics theory ever! [from: particleadventure.org] (The Periodic Table - just like Chemistry, but 16 different elementary particles much simpler and way cooler!) have been observed in collider experiments: 12 “matter particles” and 4 “force particles” Matter particles are further divided into leptons and quarks There are 6 leptons and 6 quarks: 3 “generations”, 2 leptons and 2 quarks in each Particles in each row (e.g. u, c and t quarks) are identical except for their masses: t is heavier than c, which is heavier than u Some Basic Properties of Matter Particles: • Each matter particle has an antiparticle, with exactly the same mass but opposite electric charge • Quarks do not exist as free objects, but only as constituents of “baryons” (a bound state of 3 quarks) and “mesons” (a bound state of a quark and an antiquark) ¯ + ¯ • Examples: p = uud, n = udd, p¯ = u¯u¯d, π = ud, . • 100’s of baryons and mesons have been observed, all can be understood as bound states of the known quarks • Most particles are unstable (decay into other particles, with lifetimes <<1 sec) • Exceptions: electron, 3 neutrinos, 2 baryons: proton (uud) and neutron (udd), and their antiparticles, are STABLE FOUR FUNDAMENTAL FORCES: Gravitational force: motion of planets, rockets, apples, ... Electromagnetic force: electricity, radiowaves, light, ... Weak force: origin of radioactivity Strong force: binds protons and neutrons in the nucleus Gravity is very weak – a small magnet can balance the gravitational effect of the entire Earth, BUT: Only one type of gravitational charge (always attractive), forces add up – very relevant over long distances (while E&M charges cancel) Modern Picture of Forces: forces between “matter particles” are due to exchange of “force particles” For example: Electromagnetic force between electrons is due to a photon exchange “Quantum Electrodynamics” - combines Maxwell’s theory of electromagnetism, Special Relativity and Quantum Mechanics Electric and magnetic forces are described by emission and absorption of photon – a particle of zero mass, the force carrier of electromagnetism Feynman “Feynman Diagram” [Nobel prize 1965, with Schwinger, Tomonaga] Weak Interactions Weak force is described by a theory just like electrodynamics, but instead of photon, mediating particles are the W and Z bosons ● Weak force is short-range, with range about 1 0 − 1 5 cm: e−r/r0 V ∝ weak r ● This implies that the W and Z bosons are massive: ¯h M ∼ ∼ 100 GeV cr0 Feynman diagram for ● Discovery of W and Z: CERN, 1983 the neutron beta-decay [Nobel prize 1984: Rubbia, van der Meer] EM-Weak Unification HERA I high Q2 e-p EM and Weak forces ) 2 • H1 e-p NC - 10 ZEUS e p NC 98-99 become equally strong SM e-p NC (CTEQ5D) (pb/GeV 2 1 at short distances of /dQ ! d -1 10 order cm -2 −15 10 10 -3 10 Same theory describes - • H1 e p CC -4 - 10 ZEUS e p CC 98-99 both forces in a unified SM e-p CC (CTEQ5D) -5 10 framework -6 10 y < 0.9 -7 10 3 4 10 10 Q2 (GeV2) [blue=EM, red=weak] [Nobel prize 1979: Glashow, Salam, Weinberg] Strong Interactions Strong force is also described by a theory very similar to electrodynamics, the force particle is the gluon Due to peculiar nature of the gluon, the strong force grows with distance between charges: V ∝ r Only quarks experience the strong force, leptons are immune to it (neutral). This explains why quarks are confined and leptons are not! [Nobel prize 2004: Gross, Politzer, Wilczek] At short distances, the strong force gets weaker - the closer together you bring the quarks, the more freedom they feel! (”asymptotic freedom”) Gravitational Interactions Gravitational force is supposedly described by a theory very similar to electrodynamics, the force particle is the graviton Just like photon is a quantum of electromagnetic wave, graviton would be a quantum of gravitational wave Gravitaional waves are predicted by General Relativity, and their indirect effects have been seen, but NO direct observation so far! LIGO gravitational wave detectors in Hanford, Washington and Livingston, Louisiana This concludes our brief tour of matter particles... and their interactions/force particles! Predictive Power of the Standard Model • The Standard Model is not just a list of particles and a classification - it is a theory that makes detailed, precise quantitative predictions! • Consider a head-on collision of a 100 GeV electron and a 100 GeV antielectron (”positron”). Possible outcomes: e+e−, µ+µ−, τ +τ −, pp¯, W +W −, e+e+e−e−, . • Quantum mechanics: there is no way to know for sure which outcome will occur in a given collision, but the SM predicts probabilities (”cross sections”) of each outcome, plus details like directions of the produced particles, etc. • Works spectacularly well! (some predictions experimentally verified to 0.1% accuracy) Anything Left to Discover? • The Standard Model has been the accepted theory of particle physics since ~1974 • No statistically significant deviations from the Standard Model predictions have been experimentally observed to date, in spite of years of effort • Most aspects of the model have already been experimentally verified • ~10 Nobel prizes have already been awarded for theoretical and experimental work related to the Standard Model • So, is particle physics over? NO! Truffle 1: The Higgs Boson • The Higgs boson: a hugely important aspect of the Standard Model still awaits experimental verification! • SM postulates that the Universe is filled with a uniform “Higgs field” (somewhat similar to electric/magnetic fields) • A massless particle moving in a Higgs field is equivalent to a massive particle Higgs field “gives mass to all particles” • Different particles have different masses due to different strength of their interaction with the Higgs field (”charge”) Where Is The Higgs Boson? • Relativity + Quantum Mechanics guarantee that any field must have a particle associated with it: e.g. electric/magnetic field photon, gravitational field graviton, etc. • Higgs field Higgs particle (or Higgs boson) • Experiments at the Tevatron and other colliders have hunted for the Higgs for years and have not found it! • Maybe this aspect of the SM is wrong! (Alternative theories of mass generation exist, although none is as appealing...) • Maybe the Higgs is just too heavy - theory only predicts a range for its mass, with upper bound well above the current sensitivity... • Need a more powerful accelerator/collider to test this! Large Hadron Collider (LHC) at CERN (Geneva, Switzerland) will turn on in Summer 2008! The LHC: 7 TeV protons (7 times more powerful than the Tevatron!), 17 miles long, few GEuros If Higgs exists, the LHC will find it! LHC cooldown status (this morning) [from http://lhc.web.cern.ch/lhc/] Truffle 2: Beyond the Higgs • It is highly unlikely that the Higgs boson will be the only new particle discovered at the LHC • It’s obvious: in a generic quantum field theory, any scalar field mass term receives quadratically divergent radiative corrections, and severe fine-tuning is required to maintain a hierarchy between the scalar mass and the cutoff of the theory (Ken Wilson, 1970s) • In other words, if the Higgs is there, something must “stabilize” it! Supersymmetry (a.k.a. SUSY) • Supersymmetry is the idea that there is a “superpartner” for each elementary particle: selectron, smuon, stau, squarks, photino, wino, higgsino, etc. • This would stabilize the Higgs (trust me!) • Many superpartners could be discovered at the LHC! However, Higgs+SUSY is not the only possible mechanism to generate masses... Alternatives include: technicolor, extended technicolor, walking technicolor, composite Higgs, large
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