Elementary Particle Physics Research Achim Geiser, DESY Hamburg Summer Student Lecture, 30.7.10 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: slight emphasis on DESY-related topics www pages of DESY and CERN 30.7.10 A. Geiser, Particle Physics 1 What is Particle Physics? 30.7.10 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) 30.7.10 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) 30.7.10 A. Geiser, Particle Physics 4 History of basic building blocks of matter 0 − − + οο Σ + π − Κ−ΔΛ+Δ motivation: πΔπΔ0 pΩ + find Κ+ Κ smallest + Super- possible number symmetry AD 30.7.10 A. Geiser, Particle Physics 5 Which Interactions? Three 30.7.10 A. Geiser, Particle Physics 6 The Forces in Nature at ~ 1 GeV 30.7.10 A. Geiser, Particle Physics 7 What we know today GGrraavviittyy g s s - upu charmc topt gluon - er er i i tthhee r γ r Quarks Quarks downd s bottomb strange photon ar ar gghhoosstt aatt C C νe νμ ντ W tthhee e-neutrino μ-neutrino τ-neutrino W boson ce ce 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 30.7.10 A. Geiser, Particle Physics 8 The Power of Conservation Laws e.g. radioactive neutron decay: - not visible n p + e + νe Pauli 1930: Wolfgang Pauli (Nobel 1945) 30.7.10 A. Geiser, Particle Physics 9 confirmation: neutrino detection e.g. reversed reaction: νe+ 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 30.7.10 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! 30.7.10 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) 30.7.10 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) - - muon decay: μ -> νμ e νe conservation of Leon M. Melvin Jack Ledermann Schwartz Steinberger electric charge -1 0 -1 0 lepton number: 1 1 1 -1 ν = ν (1955) „muon number“: 1 1 0 0 νμ = νe (1962) There is a distinct neutrino for each charged lepton 30.7.10 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 (light) neutrino ν flavours = 3 Gerardus Martinus t’Hooft Veltman (Nobel 1999) e+e- -> Z0 30.7.10 A. Geiser, Particle Physics 14 Can we “see” particles? Luis Walter Alvarez (Nobel 1968) we can! bubble chamber photo Donald Arthur Glaser (Nobel 1960) 30.7.10 A. Geiser, Particle Physics 15 A generic modern particle detector 30.7.10 A. Geiser, Particle Physics 16 Particle Physics = People 30.7.10 A. Geiser, Particle Physics 17 Why do we need colliders? early discoveries in cosmic rays, but need controlled Mont Blanc conditions V.F. Hess E (Nobel 1936) m = c2 Albert Einstein CERN (Nobel 1921) need high energy to discover new heavy particles LEP/LHC colliders = microscopes (later) 30.7.10 A. Geiser, Particle Physics 18 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) 30.7.10 A. Geiser, Particle Physics 19 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) Murray S=-1 Gell-Mann s (Nobel 1969) 30.7.10 A. Geiser, Particle Physics 20 Colour Quark model very successful, but seems to violate ++ quantum numbers (Fermi statistics), e.g. Δ =↑↑↑uuu => introduce new degree of freedom: q g g q q gggg qq g g q 3 coulours -> SU(3)colour qqq = qq = white! 30.7.10 A. Geiser, Particle Physics 21 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 30.7.10Sin-Itoro Julian Richard P. A. Geiser, Particle Physics 22 Tomonaga Schwinger Feynman Anti-Screening of Coulour Charge! quark-antiquark pairs -> screening gluons carry colour -> gg pairs -> anti-screening! (Nobel 2004) asymptotic confinement freedom 1/r2~E2, 30.7.10 A. Geiser, Particle Physics 23 Measurements of the Running coupling αs (HERA) (LEP, PETRA) 30.7.10 A. Geiser, Particle Physics 24 calculation of proton mass in QCD from lattice gauge theory: p spontaneous breakdown of “chiral symmetry” (left-right-symmetry) Yoichiro yields QCD “vacuum” expectation value Nambu => proton mass (Nobel 2008) => most of the visible mass of the universe! 30.7.10 A. Geiser, Particle Physics 25 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 2-jet Trajectories of charged particles reconstructed in drift chambers event (gas ionisation detectors). (Nobel 1992) Photons - dotted lines - detected by lead-glass Cerenkov counters. Two opposite jets. 30.7.10 A. Geiser, Particle Physics 26 Discovery of the Gluon (1979) q PETRA at DESY: look for t γ g Björn Wiik Paul Söding q 3-jet event TASSO event picture Günter Wolf Sau Lan Wu (EPS prize 1995) 30.7.10 A. Geiser, Particle Physics 27 How to resolve the structure of an object? scattering image e.g. X-rays probe (Hasylab, FLASH, accelerator PETRA III, XFEL) E~ keV -> structure of a biomolecule 30.7.10 A. Geiser, Particle Physics 28 Howto resolvethestructureof theproton? microscope: resolution ~ 10-18 m = 1/1000 of size of a proton low resolution -> small instrument high resolution -> large instrument Data taking -> summer 2007. Data analysis -> 2014. 30.7.10 A. Geiser, Particle Physics 29 Inside the proton Low Q2 (large λ) Jerome I. Henry W. Richard E. 2 Medium Q (medium λ) Friedmann Kendall Taylor (Nobel 1990) Large Q2 (short λ) At higher and higher resolutions, the quarks emit gluons, which also emit gluons, which emit quarks, which……. 2 -18 30.7.10At highest Q A. ,Geiser, λ ~ 1/QParticle ~ Physics 10 m 30 Example: Higgs cross section at LHC gluons in p from HERA -> predict Higgs at LHC 30.7.10 A. Geiser, Particle Physics 31 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” mass generated by Higgs field 30.7.10 A. Geiser, Particle Physics 32 Discovery of the W and Z (1983) To produce the heavy W and Z bosons (m ~ 80-90 GeV) need high energy collider! 1978-80: conversion of SPS proton accelerator at CERN into proton-antiproton collider challenge: make antiproton beam! success! Z0 -> e+e- UA1 -> first W and Z produced 1982/83 (Nobel 1984) Simon Carlo van der Rubbia Meer 30.7.10 A. Geiser, Particle Physics 33 Electroweak Physics at HERA Neutral Current (NC) interactions e Charged Current (CC) interactions e 30.7.10 A. Geiser, Particle Physics 34 Weak interactions are "left-handed" lefthanded electrons interact (CC) Polarized CC Cross Sections e- righthanded electrons do not! e- cross section linearly proportional to polarization left polarization e± p e± p right σ polCC = (1± Pe)⋅σ unpolCC 30.7.10 A. Geiser, Particle Physics 35 Electroweak Unification NC CC 2 MW 30.7.10 A. Geiser, Particle Physics 36 The Quest for Unification of Forces Electroweak Unification Grand Unified Theories ? Superstring Theories ? Maxwell’s equations strong Big Bang electric magnetic weak gravity 30.7.10 A. Geiser, Particle Physics 37 Antimatter relativistic Schrödinger equation P.A.M. (Dirac equation) Dirac two solutions: (Nobel 1933) one with positive, one with negative energy Dirac: interpret negative solution as 1932 antielectrons (positrons) found in conversion of energy into matter C.D.Anderson (Nobel 1936) 1995 antihydrogen consisting of antiprotons and positrons produced at CERN In principle: antiworld can be built from antimatter In practice: produced only in accelerators and in cosmic rays 30.7.10 A.