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Particle Physics what do we know? Ulrich Heintz Boston University 8/5/2002 Ulrich Heintz - Quarknet 2002 1 Particle Physics • What associations does the word particle physics bring to your mind? 8/5/2002 Ulrich Heintz - Quarknet 2002 2 Particle Physics • What are the fundamental building blocks of the universe? • What are the interactions between them? • How can we explain the universe? – its history – its present form – its future • Is there a theory of everything? 8/5/2002 Ulrich Heintz - Quarknet 2002 3 Particle Physics it’s fun and fascinating 8/5/2002 Ulrich Heintz - Quarknet 2002 4 What is a particle? • a small piece of matter... • characterized by – – – – charge mass lifetime spin • particles can scatter off each other like billiard balls • unlike billiard balls, most particles are unstable and decay • particles can be produced by colliding other particles 8/5/2002 Ulrich Heintz - Quarknet 2002 5 What was the world made of in 1932? • electrons (1897) – orbit atomic nucleus • proton (1911) – nucleus of lightest atom • neutron (1932) – neutral constituent of the nucleus • photon (1905) – quantum of the electromagnetic field 8/5/2002 Ulrich Heintz - Quarknet 2002 6 and... • 1927 Dirac’s relativistic quantum mechanics • 1931 the positive electron (positron) – antiparticles: for every particle there exists an antiparticle with same mass, lifetime, spin, but opposite charge • 1930 Pauli’s neutrino – energy conservation in beta decay requires the existence of a light, neutral particle – n p+ + e- + – observed in 1956 • 1936-1947 the muon and the pions (+,0,-) – Rabi: “who ordered that?” 8/5/2002 Ulrich Heintz - Quarknet 2002 7 The ascent of accelerators • previous discoveries used – cosmic rays – “natural accelerators” (radioactivity) • after WWII – accelerators 8/5/2002 Ulrich Heintz - Quarknet 2002 8 The particle “Zoo” • 1947: strange particles – K0+ -, K++ + – p+ – , – long lifetime ¼ 10-10 s • more particles... – p, – – short lifetime ¼ 10-24 s 8/5/2002 Ulrich Heintz - Quarknet 2002 9 The quark model • 1964 Gell-Mann, Zweig – there are three quarks and their antiparticles Quark Charge Up +2/3 Down -1/3 Strange -1/3 – each quark can carry one of three colors • red blue green – antiquarks carry anticolor • anti-red anti-blue anti-green 8/5/2002 Ulrich Heintz - Quarknet 2002 10 The quark model – only colorless (“white”) combinations of quarks and antiquarks can form particles • qqq • qq • no others observed 8/5/2002 Ulrich Heintz - Quarknet 2002 11 The 8-fold way baryons qqq mesons qq K0 K+ ds us - 0 + ++ ddd udd uud uuu n p 0 dds - ud uu,dd,ss uds dss uss - su sd K- K0 8/5/2002 uus + 0 du + 0 sss Ulrich Heintz - Quarknet 2002 12 Quark confinement • What holds quarks/antiquarks together? – strong force – acts between all “colored” objects – short range – independent of distance 8/5/2002 Ulrich Heintz - Quarknet 2002 13 So what is the world made of? The Standard Model e e u d 0.511 MeV 0 a few MeV a few MeV spin = ½ (fermions) 8/5/2002 c s 106 MeV 0 1100 MeV 150 MeV t b 1.8 GeV 0 175 GeV 4.2 GeV leptons quarks Ulrich Heintz - Quarknet 2002 14 Are these fundamental? • As far as we know.... – we can measure structure as small as 10-18 m • Accelerators are like huge microscopes – To measure smaller distances – go to higher energies 8/5/2002 Ulrich Heintz - Quarknet 2002 15 How do particles interact? • particles attract or repel each other by exchanging “messenger” particles (field quanta) e e 8/5/2002 Ulrich Heintz - Quarknet 2002 Feynman diagram 16 What holds the world together? strong electromagnetic weak gravity quarks charged particles all particles all particles relative strength 10 10-2 10-13 10-42 field quantum g W§ Z0 G force acts between spin = 1 (bosons) 8/5/2002 Ulrich Heintz - Quarknet 2002 17 The Higgs boson • the standard model requires the existence of one more particle • Higgs boson – uncharged – unknown mass (>115 GeV) – spin = 0 • required to be able to describe massive fermions and bosons 8/5/2002 Ulrich Heintz - Quarknet 2002 18 Is this the theory of everything? • NO – Standard Model doesn’t work at all energies – Standard Model does not include gravity – we haven’t found the Higgs yet... • unification Electricity Magnetism Weak force electromagnetism electroweak force Strong force GUTs Gravity 8/5/2002 string theory... Ulrich Heintz - Quarknet 2002 19 Accelerators • 1983: CERN pp collider – E = 540 GeV W§ (80 GeV), Z0 (91 GeV) • 1995: Fermilab Tevatron pp collider – E=1.8 TeV top quark (175 GeV) • ¼ 2008: CERN LHC pp collider – E=14 TeV discover Higgs? • ????: Linear e+e- Collider – E=1-2 TeV study Higgs in detail 8/5/2002 Ulrich Heintz - Quarknet 2002 20 What might we find? • Super Symmetry – fermions bosons electron selectron neutrino sneutrino quark squark photon photino gluon gluino W Wino Z Zino – we have already found half the particles.... 8/5/2002 Ulrich Heintz - Quarknet 2002 21