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particle - Concepts in Theoretical Physics Lecture 6...

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Concepts in Theoretical Physics David Tong Lecture 6: Particle Physics
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The Structure of Things • Four fundamental particles • Four fundamental forces • All based on symmetry which means group theory • The Higgs boson + stuff we don’t know + stuff that we don’t even know we don’t know • Repeated twice! ν e d u
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The New Periodic Table electron down quark up quark strange quark charm quark bottom quark top quark neutrino muon neutrino tau neutrino muon tau Electric charge 0 -1/3 2/3 -1 ~200 ~3000 ~10 -6 ~4 ~8 ~200 ~8000 ~340,000 ~2000 electron mass = 9.10938188 × 10 -31 kilograms 1 Mass ~10 -6 ~10 -6
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+ Anti-Particles Each type of particle has an anti-particle This has the same mass, but opposite charge If a particle and anti-particle come across each other, they annihilate. Anti-particles were predicted in 1930 by Dirac, and discovered 2 years later ( i γ μ D μ m ) ψ = 0
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The Zoo of Particles The proton is made of two up quarks and a down quark The neutron is made of two down quarks and an up quark In the 1960 s, physicists discovered hundreds upon hundreds of further particles. Each contains either three quarks, or a quark and an anti-quark There are pions and kaons and vector rho mesons. There’s the Delta ++ baryon and the omega, lambda, sigma, eta, nu, upsilon. After running out of greek letters, they were named simply a, b, f
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How to Make Stuff Why do the quarks stick together in this way? It s because the quarks are the only particles to feel the strong nuclear force. To understand this better, we next need to look at the forces.
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Four Forces The particles interact with each other through four forces: Electromagnetism (QED) Strong Nuclear Force (QCD) Weak Nuclear Force Gravity We don t understand gravity as well the others see the next lecture Each of these comes with an associated particle which transmits the force photon W and Z boson gluons Not discovered, although there is strong evidence for gravity waves
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