Elementary Particle Physics Research Achim Geiser, DESY Hamburg Summer Student Lecture, 3.8.06 Scope of this lecture: Introduction to particle physics for non-specialists rather elementary more details -> specialized lectures particle physics in general thanks to B. Foster for some of the nicest slides/animations other sources: some emphasis on DESY-related topics www pages of DESY and CERN 3.8.06 A. Geiser, Particle Physics 1 What is Particle Physics? 3.8.06 A. Geiser, Particle Physics 2 What is a „particle“? Classical view: particles = discrete objects. energy concentrated into finite space with definite boundaries. Particles exist at a specific location. -> Newtonian mechanics Isaac Newton Modern view: particles = objects with discrete Niels quantum numbers, e.g. charge, mass, ... Bohr not necessarily located at a specific position. (Nobel 1922) (Heisenberg uncertainty principle) can also be represented by wave functions (Quantum mechanics, particle/wave duality) Louis Werner Erwin de Broglie Heisenberg Schrödinger (Nobel 1933) (Nobel 1929) (Nobel 1932) 3.8.06 A. Geiser, Particle Physics 3 What is „elementary“? Greek: atomos = smallest indivisible part Dmitry Ivanowitsch Mendeleyev 1868 (elements) Ernest Rutherford 1911 (nucleus) (Nobel 1908) Murray Gell-Mann 1962 (quarks) (Nobel 1969) 3.8.06 A. Geiser, Particle Physics 4 History of basic building blocks of matter 0 − − + οο ΚΣ∆Λ+∆π − ∆− + Ω motivation: ππ∆0 p + find Κ+ Κ smallest + Super- possible number symmetry AD 3.8.06 A. Geiser, Particle Physics 5 Which Interactions? Three 3.8.06 A. Geiser, Particle Physics 6 The Forces in Nature at ~ 1 GeV 3.8.06 A. Geiser, Particle Physics 7 What we know today GGrraavviittyy g s u c t s -- r up charm top gluon r e e i γ i tthhee r r r r Quarks Quarks downd s bottomb strange photon a a gghhoosstt aatt C ν ν ν C e µ τ tthhee e W e e-neutrino µ-neutrino τ-neutrino c W boson c r r ffeeaasstt o e µ τ Z o Leptons F Leptons electron muon tau Z boson F II IIII IIIIII HiggsHiggs GGeenneerraattiioonnss ooff BosonBoson?BosonBoson? mmaatttteerr 3.8.06 A. Geiser, Particle Physics 8 The Power of Conservation Laws e.g. radioactive neutron decay: - ν not visible n p + e + e Pauli 1930: Wolfgang Pauli (Nobel 1945) 3.8.06 A. Geiser, Particle Physics 9 confirmation: neutrino detection e.g. reversed reaction: ν e+ n p + e extremely rare! Frederick Reines (absorption length ~ 3 light years Pb) (Nobel 1995) first detection: 1956 (!) Reines and Cowan, neutrinos from nuclear reactor 3.8.06 A. Geiser, Particle Physics 10 The power of symmetries: Parity Will physical processes look the same when viewed through a mirror? In everyday day life: violation of parity symmetry is common „natural“: our heart is on the left „spontaneous“: cars drive on the right Eugene (on the continent) Wigner What about basic interactions? (Nobel 1963) Electromagnetic and strong interactions conserve parity! 3.8.06 A. Geiser, Particle Physics 11 The power of symmetries: Parity Lee & Yang 1956: weak interactions violate Parity experimentally verified by Wu et al. 1957: Chen Ning Yang (Nobel 1957) spin Tsung -Dao consequence: Lee neutrinos are always Chieng Shiung lefthanded ! Wu (antineutrinos righthanded) 3.8.06 A. Geiser, Particle Physics 12 The Power of Quantum Numbers 1948: discovery of muon Who ordered THAT ? same quantum numbers as electron, except mass (Nobel 1988) I.I. Rabi (Nobel 1944) µ- ν - ν muon decay: -> µ e e conservation of Leon M. Melvin Jack Ledermann Schwartz Steinberger electric charge -1 0 -1 0 lepton number: 1 1 1 -1 ν = ν (1955) ν ν „muon number“: 1 1 0 0 µ = e (1962) There is a distinct neutrino for each charged lepton 3.8.06 A. Geiser, Particle Physics 13 The Power of Precision Precision measurements of shape and height of Z0 resonance at LEP I (CERN 1990’s) ν number of ν ν (light) neutrino flavours = 3 Gerardus Martinus t’Hooft Veltman (Nobel 1999) e+e- -> Z0 3.8.06 A. Geiser, Particle Physics 14 Can we “see” particles? Luis Walter Alvarez (Nobel 1968) we can! bubble chamber photo Donald Arthur Glaser (Nobel 1960) 3.8.06 A. Geiser, Particle Physics 15 A generic modern particle detector 3.8.06 A. Geiser, Particle Physics 16 Why do we need colliders? early discoveries in cosmic rays, but Mont Blanc need controlled conditions E m = c 2 CERN need high energy to discover new heavy particles colliders = LEP/LHC microscopes (later) 3.8.06 A. Geiser, Particle Physics 17 The HERA ep Collider and Experiments 3.8.06 A. Geiser, Particle Physics 18 Particle Physics = People 3.8.06 A. Geiser, Particle Physics 19 Strong Interactions: Quarks and Colour strong force in nuclear interactions = „exchange of massive pions“ between nucleons = residual Van der Waals-like interaction Hideki Yukawa (Nobel 1949) n modern view: π (Quantum Chromo-Dynamics, QCD) exchange of massless gluons p between quark constituents „similar“ to electromagnetism (Quantum Electro-Dynamics, QED) 3.8.06 A. Geiser, Particle Physics 20 The Quark Model (1964) arrange quarks (known at that time) into flavour-triplett => SU(3)flavour symmetry Q=-1/3 Q=2/3 d u treat all known hadrons S=0 (protons, neutrons, pions, ...) as objects composed of two or three such quarks (antiquarks) S=-1 s 3.8.06 A. Geiser, Particle Physics 21 The Quark Model baryons = qqq mesons = qq 3.8.06 A. Geiser, Particle Physics 22 Colour Quark model very successful, but seems to violate ++ quantum numbers (Fermi statistics), e.g. ∆ =uuu ↑↑↑ => introduce new degree of freedom: q g g q q gggg qq g g q 3 coulours -> SU(3)colour qqq = qq = white! 3.8.06 A. Geiser, Particle Physics 23 Screening of Electric Charge electric charge polarises vacuum -> virtual electron positron pairs positrons partially screen electron charge effective charge/force (Nobel 1965) decreases at large distances/low energy (screening) increases at small distance/large energy 3.8.06Sin-Itoro Julian Richard P. A. Geiser, Particle Physics 24 Tomonaga Schwinger Feynman Anti-Screening of Coulour Charge! quark-antiquark pairs -> screening gluons carry colour -> gg pairs -> anti-screening! asymptotic confinement freedom 1/r2~E2, 3.8.06 A. Geiser, Particle Physics 25 3.8.06 A. Geiser, Particle Physics 26 Comparison QED / QCD electromagnetism strong interactions QED QCD 1 kind of charge (q) 3 kinds of charge (r,g,b) force mediated by photons force mediated by gluons photons are neutral gluons are charged (eg. rg, bb, gb) α α is nearly constant s strongly depends on distance confinement limit: The underlying theories are formally almost identical! 3.8.06 A. Geiser, Particle Physics 27 The effective potential for qq interactions confinement asymptotic freedom 3.8.06 A. Geiser, Particle Physics 28 Burton Heavy Quark Spectroscopy Richter Charmonium = bound system (Nobel 1976) + - of cc quark pair Positronium = bound e e system Samuel C.C. Ting 1974 3.8.06 A. Geiser, Particle Physics 29 How to detect Quarks and Gluons? Jets! hadrons Example of the hadron production in e+e- q e+ e- annihilation in the JADE q detector at the PETRA e+e- collider at DESY, hadrons Germany. Georges cms energy 30 GeV. Charpak Lines of crosses - reconstructed trajectories in drift chambers (gas ionisation detectors). (Nobel 1992) Photons - dotted lines - detected by lead-glass Cerenkov counters. Two opposite jets. 3.8.06 A. Geiser, Particle Physics 30 Discovery of the Gluon (1979) PETRA at DESY: look for Björn Wiik Paul Söding TASSO event picture Günter Wolf Sau Lan Wu 3.8.06 A. Geiser, Particle Physics 31 Jets in ep interactions (HERA) 3.8.06 A. Geiser, Particle Physics 32 α Runnning coupling s from jet production HERA (E,Q ~ 1/r) 3.8.06 A. Geiser, Particle Physics 33 α Running coupling s from other measurements (HERA) (LEP, PETRA) HERA = currently best place to study QCD! 3.8.06 A. Geiser, Particle Physics 34 How to determine the „size“ of a particle? microscope: resolution ~ 10-18 m = 1/1000 of size of a proton low resolution -> small instrument high resolution -> large instrument 3.8.06 A. Geiser, Particle Physics 35 How to resolve the structure of an object? scattering image e.g. X-rays probe (Hasylab) E~ keV accelerator -> structure of a biomolecule 3.8.06 A. Geiser, Particle Physics 36 Resolvethestructureof theproton E ~ MeV resolve whole proton static quark model, Jerome I. Henry W. Richard E. Friedmann Kendall Taylor valence quarks (Nobel 1990) (m ~ 350 MeV) E ~ mp ~ 1 GeV resolve valence quarks and their motion E >> 1 GeV resolve quark and gluon “sea” 3.8.06 A. Geiser, Particle Physics 37 Inside the proton Heisenberg’s UP Low Q2 (large λ) allows gluons, and qq Medium Q2 (medium λ) pairs to be produced for a very short time. Large Q2 (short λ) At higher and higher resolutions, the quarks emit gluons, which also emit gluons, which emit quarks, which……. 2 -18 3.8.06At highest A. Geiser, Q , Particleλ ~ 1/Q Physics ~ 10 m 38 Deep Inelastic ep Scattering at HERA q p p remnant e e 3.8.06 A. Geiser, Particle Physics 39 Deep Inelastic Scattering (DIS) 3.8.06 A. Geiser, Particle Physics 40 The Proton Structure structure functions quark and gluon densities 3.8.06 A. Geiser, Particle Physics 41 Kinematic regions: HERA vs. LHC proton structure measured directly for large part of LHC phase space QCD evolution LHC Tevatron successful -> safely extrapolate to high Q2 or low x HERA fixed target 3.8.06 A. Geiser, Particle Physics 42 Example: Higgs cross section at LHC 3.8.06 A. Geiser, Particle Physics 43 Weak Interactions The Theory of GLASHOW, SALAM and WEINBERG ~ 1959-1968 (Nobel 1979) Theory of the unified weak and electromagnetic interaction, transmitted by exchange of “intermediate vector bosons” 3.8.06 A.