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Particle Physics

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  1. Particle Physics Physics 735: Fall 2008 Sridhara Dasu

  2. Physics 735: Goals • Gain sufficient proficiency in particle physics phenomena and phenomenology, so that • experimentalists can begin research • Tevatron or LHC • Neutrino physics • theorists can build on the foundation provided • Phenomenology • Cosmology • String theory

  3. Course Structure • Lectures • Provide introduction to the subject • Provide motivation for you to read further • Provide opportunity to discuss the subject with your fellow students and me • Please feel free to stop me and ask questions • Hopefully, the answers provided will improve your understanding of the subject and your colleagues’ • Homework (3 hours per week) • Read text books • Work out problems

  4. Lecture Logistics • Particle physicists are by necessity globe trotters – my labs are in Stanford & Geneva • I will be away for 6 of 29 lectures • I would like to compensate by longer lectures • Say, 12:30 – 14:15, Tue/Thu • I will provide some refreshments so you can stand the long lecture  • Let me know if it does NOT work and I will try to find some other way

  5. Other Details • Office hours • Tue and Wed afternoons • Call me 608-262-3678 or 408-829-6625 to ensure I am in my room • Grading • Homework (75%) • You may work in groups • You may consult me in my office hours • Final paper (25%) • You must chose and complete a project independently • Example projects will be provided later •

  6. The Quantum Universe • The big questions • What is the nature of the universe and what is it made of? • What are matter, energy, space and time? • How did we get here and where are we going? • “In the last 30 years, physicists have achieved a profound understanding of the fundamental particles and the physical laws that govern matter, energy, space and time … the Standard Model can truly be celebrated as one of the great scientific triumphs of the 20th century.” • • In Physics 735 we will study the Standard Model

  7. Quantum Universe Unified Forces • Are there undiscovered principles of nature: new symmetries, new physical laws? • How can we solve the mystery of dark energy? • Are there extra dimensions of space? • Do all forces become one? The Particle World • Why are there so many kinds of particles? • What is dark matter? How can we make it? • What are neutrinos telling us? The Birth of the Universe • How did the Universe come to be? • What happened to the antimatter?

  8. From Quarks to Cosmos • Connecting Quarks with the Cosmos, Eleven Science Questions for the New Century • National Research Council of the National Academies • ISBN 0-309-07406-1, and

  9. Reverse Engineering the Universe • A great achievement of the 20th century physics • Unification of the theory of microcosm and the macrocosm • Cosmology: The Big Bang and The Standard Model of elementary particles. • Observations • Our Universe is expanding • The relic radiation now at microwave energies observed - being studied • Our Universe is made of matter (as opposed to anti-matter!) • The Standard Model does allow matter-antimatter asymmetry • Our Universe is predominantly made of dark matter • Not enough matter in the universe to account for the rate of expansion • In the early universe just after the Big Bang the temperatures were high and matter-antimatterradiation field transitions were predominant • As the Universe cooled to below 2Melectron these transitions stopped • There must have been some asymmetry that led to matter domination • There must have been some process that caused high mass particles to decay into very weakly interacting particles (dark matter) • High energy experiments can probe phenomena relevant for cosmology • Study the physics of the early universe  Study particles predominant at ~t0. • Reasons for matter-antimatter asymmetry (quark mixing and neutrino mixing) • Discover the dark matter

  10. Particle Physics • Theoretical Framework • The Standard Model of Particle Physics • Relativistic Quantum Gauge Field Theories of microscopic frontier that is accessible to the experiments • Relativity and Quantum Mechanics folded together • But, SM cannot be complete, what is the new physics that lies beyond the SM (BSM)? • Laboratory Experimental Probes • Electro-weak Physics • Heavy quark Physics • Neutrino Physics

  11. The smallness of the electron • At the end of 19th century • Physicists pondered about the electron • Electron is point-like • At least smaller than 10-17 cm • Like charges repel • Hard to keep electric charge in a small pack • Need a lot of energy to keep it small!

  12. Energy and matter are related Energy can be transformed to mass and vice versa Conservation of mass-energy Measured energy of the electron is only 0.5 MeV Can explain a size of 10-10 to 10-13 cm Cannot explain < 10-17 cm as measured Need LOTS of energy to pack charge tightly inside the electron Breakdown of theory of electromagnetism E=mc2

  13. Uncertainty Principle: You can violate energy conservation but only for a short time Uncertainty Principle Werner Heisenberg

  14. Relativistic Quantum world • Dirac formulated Relativistic Quantum Mechanics • Schrodinger equation • Not relativistic (space2 but time1) • Predicted antimatter • Anderson discovered positron • You can create more massive objects than you have energy - but they are virtual - i.e., they disappear promptly and rematerialize in particle states that conserve mass-energy • Vacuum is full of quantum bubbles! Paul Adrian Maurice Dirac

  15. Electron creates a force to repel itself Vacuum bubble of matter anti-matter creation/annihilation Electron annihilates the positron in the bubble  Size of the electron is no longer a relevant parameter - the closer you probe, the more you see the structure of vacuum … matter and antimatter pairs Electron Stability Requires Anti-Matter

  16. Structure of Matter • Rutherford Scattering: Discovery of nucleus

  17. Finite Sized Nucleons • Hofstadter • Elastic electron scattering • At Stanford

  18. Yukawa’s Prediction of Mesons • What is responsible for strong binding between nucleons? • Yukawa (1935) postulated a new potential which is large at short distances and decreases rapidly at distances larger than about 2 fm. • Treated the problem in a relativistic quantum theory • He clearly showed that in the relativistic quantum world particles interact by exchanging virtual quanta which mediate the force • He predicted the mass of pions

  19. Yukawa’s Prediction • Following electromagnetism for new potential

  20. Who ordered the muon? • In 1937 a new particle of mass 105.7 MeV was discovered • However, it interacted with matter very weakly, a heavy lepton • Created in upper reaches of the atmosphere many of them were able to reach the ground level traversing a large amount of matter • Muon - I. Rabi asked, “Who ordered the muon?” • Yukawa’s mesons (pions) were eventually discovered in 1947

  21. Discrete Symmetries

  22. Symmetries and Quantum Numbers • Strong interactions seem to be independent of nucleon flavor (proton or neutron) • This symmetry for strong interaction implies a conserved current or quantum number (Noether’s theorem) • Isospin • Proton = +1/2 Iz, Neutron = -1/2 Iz • Isospin follows spin angular momentum algebra • Weak interactions do not conserve isospin • Neutron beta decay, np e e

  23. Many mesons and baryons • Pions, (0, ±) • Strong binding is independent of proton/neutron numbers • Isospin symmetry implied three I=1 Yukawa pions • Angular excitations, vector mesons (, , …) • Kaons, (KS, KL, K±) • Strange particles produced in pairs • Strong and EM interactions conserve strangeness a new quantum number • But weak interactions violate strangeness • Kaons decay to pions and leptons • Organizing the mesons and baryons • Flavor (softly broken) symmetries • Gellman’s eight fold way  substructure (quarks) • SU(3) symmetry invoked to explain octet of pseudo scalar mesons • Predicted missing member of decuplet of baryons, which was discovered • However, predicted fractionally charged quarks were not observed

  24. Down to Quarks Rutherford Scattering with high energy electrons Stanford Linear Accelerator Center

  25. Proton Structure Revealed

  26. Discovery of Charm

  27. Quark Model • With u, d, s and c quarks

  28. Mesons • Many types • Many decay modes • Some are long lived,i.e., > 10–8 s • Massive  short life • Detection • Long lived • Interactions with detector matter • Short lived • Calculating combined masses using detected particles

  29. Quarks and Color • Overwhelming evidence for nucleon and meson substructure in terms of quarks • Quarks are spin-1/2 and fractionally charged • However, quarks were never observed directly • Some thing confines them into mesons (qq) and baryons (qqq) • Baryons should have antisymmetric wave function • Proton, p = uud, neutron, n=udd are OK • How about, ++=uuu? • Solve both problems • Invent a new quantum number color • All particles are color less: q, qq, … cannot exist • Overcome statistics problem by choosing p=uRuGdB, ++=uRuGuB • This seemingly contrived solution is actually the scheme chosen by nature! Rather than Yukawa’s theory, the color dynamics works

  30. Screening of Electric & Color Charge

  31. Weak Interactions: Mass • Why are weak interactions (-decay) weak? • Massive mediating bosons (W± and Z0) unlike the case of photons (80, 90 GeV) - these bosons were discovered at CERN in 80s • Weak interactions change flavor • There are no large flavor changing neutral currents, i.e., not Z • Mediating bosons that change flavor are charged, W± • Flavor changing weak interactions allow decay of heavier generation quarks and leptons to lighter generations • Flavor changing neutral current transitions indicate new physics • Electroweak unification at high mass scales • Electroweak theory predicts existence of a fundamental scalar higgs and neutral currents • Interactions of W and Z with higgs field give them mass • Secondary benefit: Yukawa like couplings of matter particles to higgs field give them mass • But, Higgs boson is yet to be found (LHC)

  32. The Last Generation • Another generation of quark, bottom or beauty, was found • Symmetry implied existence of a top quark which was eventually found few years ago • Bottom forms mesons but top is so heavy that it decays before QCD can confine it to a meson • Are there more generations? • Leptons also form doublets, (e, e), (, ), (, ) • Neutrinos are almost massless • Neutrinos couple to Z boson weakly • Z decay width can predict how many neutrinos are allowed if they are less than half the mass of Z (MZ=91GeV) • Measurements indicated only 3 allowed light neutrinos • Therefore, assuming that any new neutrinos are light there are only three lepton generations of the type we know • The suggestive symmetry between lepton and quark content implies that we probably arrived at the last generation, unless the fourth generation behaves very differently from the other three

  33. Matter Particles As Dirac Predicted all of these matter particles also have corresponding antimatter particles

  34. Interactions Between Particles CERN - Geneva

  35. Particle Interactions

  36. e- b c g,Z t b e- e+ W+ W- n Elementary particles Heavier elementary particles decay - only the first generation (e,u,d), photons (g) and neutrinos (n) are stable.

  37. c b e- W- n Flavor Changing Interactions • Charged W± particles (like photons but massive - 80 GeV) change flavor of quarks • For short period energy conservation can be violated to create virtual heavy W± particles • Heavier quarks, leptons decay to lighter generations • Only u, d, electron, neutrinos remain • But, why not positrons, …? • Cross generational coupling exists • b quark decays to c quark + X • The down-type quarks mix together • Quantum mechanical superposition of states

  38. Quark Mixing Matrix

  39. Baryon Number Free quarks cannot exist - they are confined to mesons or baryons.

  40. Neutrino Mixing • Measure neutrinos from Sun (low energy) and those produced by meson decays in atmosphere (higher energy) • Well predicted flux and ratios

  41. Neutrino Mixing Matrix • Experiments are on to measure neutrino mixing matrix and absolute mass scale for neutrinos • Is there CP violation in neutrino sector?

  42. Mixing Solutions Two mass difference squares measured with large mixing angles Quark mixing is much smaller but it violates CP What is the level of 1-3 mixing in neutrino sector? Is CP violated in the neutrino sector?

  43. More on higgs • The Standard Model is not complete • In higher orders of perturbation theory QFT result in divergences, which are tamed by renormalization • Higher order corrections to higgs results in a divergence that requires high degree of tuning • One can avoid fine tuning if there are cancellations • Note corrections due to fermion-higgs and boson-higgs couplings are opposite in sign • If the particle content of SM is duplicated by including a boson partner for each fermion and a fermion partner for each boson, the cancellations will result in stable higgs mass • SUPERSYMMETRY • Super partners are not found yet - they must be massive • Side effect: if there is a conserved quantum number associated with SUSY (R-parity), then lightest SUSY particle must be stable • A candidate for dark matter • Massive stable weakly interacting neutral particle

  44. The Big Picture • Particle Physics, that can be explored in the coming decade, holds the key to at least the first two items