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Overview of Particle Physics -- the path to the Standard Model

Overview of Particle Physics -- the path to the Standard Model. Topics. historical flashback over development of the field “prehistory” 19 th century electron, radioactivity, nucleus cosmic rays spectroscopy era collider era standard model of particle physics.

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Overview of Particle Physics -- the path to the Standard Model

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  1. Overview of Particle Physics -- the path to the Standard Model

  2. Topics • historical flashback over development of the field • “prehistory” 19th century • electron, radioactivity, nucleus • cosmic rays • spectroscopy era • collider era • standard model of particle physics

  3. A Century of Particle Physics J.J Thomson Top quark 1995 Electron – 1897

  4. Sizes and distance scales • visible light: wavelength ≈5∙10-7m • virus 10-7m • molecule 10-9m • atom 10-10m • nucleus 10-14m • nucleon 10-15m • quark <10-18m

  5. The Building Blocks of a Dew Drop • dew drop: 1021 molecules of water. • Each molecule = one oxygen atom and two hydrogen atoms (H2O). • Atom: nucleus surrounded by electrons. • Electrons bound to the nucleus by photons • nucleus of a hydrogen atom = single proton. • Proton: three quarks, held together by gluons just as photons hold the electron to the nucleus in the atom   

  6. Very early era (19th century) • chemistry, electromagnetism • discharge tubes, “canal rays”, “cathode rays” • photoelectric effect (Hertz, 1887) • radioactivity (Becquerel, 1895) • X-rays (Röntgen, 1895)

  7. Atoms, Nucleus • electron: first hint that atom not indivisible • natural radioactivity  understanding of composition of atom, nucleus • atom = nucleus surrounded by electrons (Geiger, Marsden, Rutherford, 1906 -1911) • hydrogen nucleus = proton, is component of all nuclei (1920) • neutron (Bothe, Becker, Joliot-Curie, Chadwick, 1930 – 1932)

  8. Cosmic rays • Discovered by Victor Hess (1912) • Observations on mountains and in balloon: intensity of cosmic radiation increases with height above surface of Earth – must come from “outer space” • Much of cosmic radiation from sun (rather low energy protons) • Very high energy radiation from outside solar system, but probably from within galaxy

  9. Cosmic rays -- “elementary” particles • new detectors (cloud chambers, emulsions) exposed to cosmic rays  discovery of many new particles • positron (anti-electron) : predicted by Dirac (1928), discovered by Anderson 1932 • muon (μ): 1937 Nedermeyer • pion (π) predicted by Yukawa (1935), observed 1947 (Lattes, Occhialini, Powell) • strange particles (K, Λ, Σ,…..

  10. Particle Zoo • 1940’s to 1960’s : • Plethora of new particles discovered (mainly in cosmic rays): • e-, p, n, ν, μ-, π±, π0, Λ0, Σ+ , Σ0 , Ξ,…. • question: • Can nature be so messy? • are all these particles really intrinsically different? • or can we recognize patterns or symmetries in their nature (charge, mass, flavor) or the way they behave (decays)?

  11. The Particle Zoo! ±, 0, e, ±, K±, K0S, K0L, 0, p, n, +, 0, , , …

  12. Particle spectroscopy era • 1950’s – 1960’s: accelerators, better detectors • even more new particles are found, many of them extremely short-lived (decay after 10-21 sec) • 1962: “eightfold way”, “flavor SU(3)” symmetry (Gell-Mann, Ne’eman) • allows classification of particles into “multiplets” • Mass formula relating masses of particles in same multiplet • quark model – three different kinds of quarks (u, d, s) • Allows prediction of new particle Ω- , with all of its properties (mass, spin, expected decay modes,..) • subsequent observation of Ω- with expected properties at BNL (1964)

  13. Ω-BNL1964 • eight-fold way  quark model – particles made up of three different “quarks” – u, d, s • p = uud, n = udd,… Ω- = sss • refinement of these ideas, more quarks, “color”, gauge field theory  Standard Model http://www.bnl.gov/bnlweb/history/Omega-minus.asp

  14. Standard Model • A theoretical model of interactions of elementary particles, based on quantum field theory • Symmetry: • SU(3) x SU(2) x U(1) • “Matter particles” • Quarks: up, down, charm,strange, top, bottom • Leptons: electron, muon, tau, neutrinos • “Force particles” • Gauge Bosons •  (electromagnetic force) • W, Z (weak, electromagnetic) • g gluons (strong force) • Higgs boson • spontaneous symmetry breaking of SU(2) • mass

  15. Contemporary Physics Education Project

  16. Particles of Standard Model Quarks Leptons +2/3 -1/3 -1 0 ne u u d d e u d I II III nm c c s s m c s BosonsFermions nt t b b t t t b g g g g g g g g g Z W±

  17. “every-day” matter Proton Neutron d u u d u d Photon g Electron Electron Neutrino e ne

  18. Electron Proton Electromagnetic interaction q1 Photon q2

  19. Proton Neutron d u u d u d Electron Anti-electron Neutrino W - e ne Weak interaction Beta decay Mean lifetime of a free neutron ~ 10.3 minutes Mean lifetime of a free proton > 1031 years!

  20. The Strong Force d u g Strong force caused by the exchange of gluons u d

  21. Forces (interactions) • Strong interaction 1 • Binds protons and neutrons to form nuclei • Electromagnetic interaction 10-2 • Binds electrons and nuclei to form atoms • Binds atoms to form molecules etc. • Weak interaction 10-10 • Causes radioactivity • Gravitational interaction 10-39 • Binds matter on large scales

  22. What holds the world together?

  23. 1977 – 1992 Many null results 1992 – 1993 A few interesting events show up 1994, DØ mt > 131 GeV/c2 1994, CDF First evidence mt ~ 170GeV/c2 1995 – CDF, DØ Discovery! The Discovery of Top Quark

  24. Creating Top Anti-Top Quark pairs

  25. Artist’s impression of a top event

  26. Muon Missing energy Electron What do we actually “see” Jet-1 Jet-2

  27. - ® m + e jets t t “event display” of a DØ top event

  28. Ωb (http://www.fnal.gov/pub/presspass/images/DZero-Omega-discovery.html • 2008 DØ experiment at Fermilab: • discover brother of Ω- , the Ωb • Ω- = sss, Ωb = ssb, • theory predicts properties, decay modes, .. • confirmed by experiment

  29. Summary • we’ve come a long way …… • Standard Model (theory of particle interactions) works embarrassingly well! • Has been tested by many hundreds of precision measurements over last three decades – very few measurements differ by more than 1 or 2 standard deviations • Even some amount of frustration – always hope to see experimental result in disagreement with theory • But there are some open questions …………………

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