Phy107Spr07Lect41 - Exam 4 Fri May 10 in-class 2 0 q u e s...

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1 Phy107 Lecture 41 1 Today: Review Particle Physics • Particles and fields: a new picture • Quarks and leptons • The weak interaction • Unification and mass • The approach of string theory • Particle physics and the universe Exam 4 : Fri. May 10, in-class 20 questions, covers fission, fusion, particle physics No final exam. Essays returned Friday Phy107 Lecture 41 2 Particles and fields • Think about electrons and photons • Modern view is that both are excitations of quantum ‘fields’ that permeate all state. • Photon is an excitation of the photon field. • Electron is an excitation of the electron field. But there is a difference: Electron is a ‘matter particle’ Photon is a ‘force particle’ Phy107 Lecture 41 3 Quantum Electrodynamics: QED • A theory of electrons and photons • Electromagnetic force arises from exchange of photons. electron electron photon Electromagnetic repulsion via emission of a photon Phy107 Lecture 41 4 Question An electron and anti-electron (positron) can annihilate, so that the particles ‘vanish’. What is left after the annihilation? A. nothing B. quarks C. photons D. neutrinos Phy107 Lecture 41 5 Pair production, annihilation • Electron and positron can ‘annihilate’ to form two photons. • Photon can ‘disappear’ to form electron-positron pair. • Relativity: Mass and energy are the same – Go from electron mass to electromagnetic/photon energy Phy107 Lecture 41 6 Electrons and photons • Electromagnetic force and electrons are both fields. • The fields have quanta: photon and electrons. • The Quantum field theory QED explains how they interact. • Very successful theory: explains perfectly all the interactions between electrons and photons • Predicted a few things we didn’t expect: – Antiparticles - the positron. – Electrons and positions can be annihilated to photons and vice versa.
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2 Phy107 Lecture 41 7 Creating more particles • All that is needed to create particles is energy. • Energy can be provided by high-energy collision of particles. An example: – Electron and positron annihilate to form a photon. – This can then create particles with mc 2 <photon energy. e- e-, μ - e+ e+, μ + First new particle found this way μ , Muon mass: 100MeV/c 2 , electron mass 0.5 MeV/c 2 Phy107 Lecture 41 8 All the leptons • Six different leptons: cats and fleas • The cats have an electrical charge of - e • The fleas (neutrinos) have zero charge (neutral). Phy107 Lecture 41 9 Three lepton generations • Each electron-like particle and neutrino paired in a generation. e ν e μ ν μ τ ν τ Generation I Generation II Generation III Phy107 Lecture 41 10 Question The difference between the different generations of leptons is A. their charge B. their mass C. their color D. their spin Phy107 Lecture 41 11 Quarks Six different kind of quarks, analogous to the six leptons All quarks have an electric charge, they couple to photon field.
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Phy107Spr07Lect41 - Exam 4 Fri May 10 in-class 2 0 q u e s...

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