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Particle Physics 2

Particle Physics 2. Topics. Rules of the Standard Model Particle Physics Experiments Beyond the Standard Model Summary. (Some) Rules of the Standard Model. Rules of the Standard Model. The Standard Model (SM) contains the detailed rules by which particles interact.

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Particle Physics 2

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  1. Particle Physics2

  2. Topics • Rules of the Standard Model • Particle Physics Experiments • Beyond the Standard Model • Summary

  3. (Some)Rules of the Standard Model

  4. Rules of the Standard Model The Standard Model (SM) contains the detailed rules by which particles interact. Many of these rules are stated in terms of quantum numbers, rather like the atomic quantum numbers, n, l, and m. However, unlike the atomic quantum numbers, we do not have a deep understanding of the quantum numbers of the SM!

  5. Quark Quantum Numbers Quark Q B c s t b u +2/3 +1/3 0 0 0 0 d -1/3 +1/3 0 0 0 0 c +2/3 +1/3 +1 0 0 0 s -1/3 +1/3 0 -1 0 0 t +2/3 +1/3 0 0 +1 0 b -1/3 +1/3 0 0 0 -1 Q – electric charge B – baryon number Quantum numbers (QN) of antiparticles = negative of QN of particles.

  6. Lepton Quantum Numbers Lepton Q L Le Lμ Lτ νe 0 1 1 0 0 e- -1 1 1 0 0 νμ 0 1 0 1 0 μ- -1 1 0 1 0 ντ 0 1 0 0 1 τ--1 1 0 0 1 Q – electric charge L – lepton number Quantum numbers (QN) of antiparticles = negative of QN of particles.

  7. Conservation Laws In the Standard Model, the following quantities are conserved in all reactions • 4-momentum • Angular momentum • Electric charge • Color charge • Baryon number • Lepton number • Electron, muon and tau numbers

  8. Conservation Laws – Examples Conserves: 4-momentum electric charge lepton number electron number muon number Conserves: 4-momentum electric charge lepton number electron number muon number

  9. Conservation Laws – Examples Conserves: 4-momentum electric charge lepton number tau number Conserves: 4-momentum electric charge lepton number electron number baryon number

  10. Conservation Laws – Examples Conserves: 4-momentum electric charge color charge Note: all gluons must be dual-colored! Conserves: 4-momentum color charge

  11. Particle Physics Experiments

  12. W+ W- What is an Event? Timescales ~ 10-24 s – 10-11 s Distance scales ~ 10-16 m – 10-3 m Energy scales (@ Fermilab) ~ 300 GeV Cross sections ~ 1010 pb – 1 pb Fermilab 1 pico-barn (pb) 10-40 m2

  13. W+ W- What is an Event? Cross sections– The SM is a quantum theory. So its predictions are typically probabilities to observe a given final state. A cross section σ is a measure of these probabilities. If L is the integrated luminosity of the colliding beams (the total number of particles that crossed a unit area) then n = σ L is the average number of times a given final stated occurred. Fermilab

  14. W+ W- What is an Event? Example– At Fermilab, the Tevatron accelerator runs at a total collision energy of 1.96 TeV. At this energy, the cross section for creating top antitop particles is ~ 8 pb. The D0 Collaboration has recorded data from ~ 5000 particles per pb (5 fb-1). Therefore, the number of top antitop final states created is ~ 40,000, of which a few thousand have been identified. Fermilab

  15. Particle Identification * Tracker * * Magnet * Tracker * * Calorimeter Magnet * * * * * * * * * * Tracker * * * * * * * * * * Photon Electron Muon Jet Neutrino

  16. Beyond the Standard Model

  17. Current Speculations Supersymmetry Technicolor Compositeness Extra dimensions Strings Brane Worlds : :

  18. Puzzles & Mysteries The Identity Puzzle What makes a top quark a top quark, an electron an electron, and a neutrino a neutrino? (Chris Quigg). In other words, what is the origin of the SM quantum numbers? The Mass Puzzle What is the origin of mass? The Matter Puzzle Why is there overwhelmingly more matter than antimatter?

  19. Puzzles & Mysteries The Just-So Puzzle Why is mass(u quark) < mass(d quark)? The Gravity Puzzle Why is gravity so weak? strong: em: weak: gravity1: 10-2: 10-5: 10-39? The Dark Matter Puzzle What is dark matter? The Dark Energy Puzzle What is dark energy?

  20. The Mass Puzzle Most of the mass of composite particles such as protons and neutrons is due to the binding energy, E, of the quarks, in accordance with Einstein’s famous formulam = E/c2. But where does the mass of the (non-composite) particles, like quarks, come from? Our current hypothesis is that space contains a quantum field, called the Higgs field, that interacts with (the massless) quark, lepton, W and Z boson fields, thereby causing the associated quanta to acquire mass. Photons and gluons remain massless. (Neutrinos are now known to have a small mass.)

  21. Proton Neutron d u u d u d The Just-So Puzzle Nucleon Mass [MeV] ____________________ N (u,d,d) 939.6 (12.25) P (d,u,u) 938.3 (9.50) ____________________ 1.3 (2.75)

  22. The Gravity Puzzle Why 10-39 ?

  23. Why is Gravity so Weak? Brane world hypothesis: Our universe is a 3-d brane embedded within an 3+n dimensional space* It is typically assumed that the brane worlds are microscopically thin * Arkani-Hamed, Dimopoulos and Dvali, Phys. Lett. B429 (1998) 263.

  24. m1 r m2 R Newton’s constant:G ~ Gbrane / Rn 3-d space Why is Gravity so Weak? Suppose that gravity can propagate a distanceR into the 3+n-dimensional void, then

  25. m1 r m2 R 3-d space Why is Gravity so Weak? According to this picture, gravity spreads out into a much larger volume than that of our 3-dimensional space. Consequently, in our 3-d space (a thin 3-d brane world), we experience only a diluted form of gravity, reflected in the small value of Newton’s constant: G ~ Gbrane / Rn.

  26. Graviton (G) Proton Proton Photon Jumping off our Brane! How can we test this idea? A proton-proton collision could create a photon recoiling against nothing, in apparent violation of 4-momentum conservation! In fact, these Zen-like events do conserve 4-momentum because the graviton would recoil against both the photon and our entire brane world.

  27. The Large Hadron Collider Geneva CERN Collision energy: 14 trillion eV(TeV) Collision rate: 600 milliontimes per second Data rate: 15 petabytes per year Temperature: ~ 1 billion x TSun’s core

  28. Summary • The Standard Model contains precise rules that govern the interactions of all known particles. • Many of these rules are given in terms of conserved quantum numbers. • Since the SM is a quantum theory, its predictions are the probabilities of observing different final states. • Although the theory is extremely successful, it leaves many fundamental questions unanswered. • But we are hopeful that data from the LHC will be able to shed light on some of these mysteries.

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