Phy107Spr07Lect39

Phy107Spr07Lect39 - Question From Last Time W h ic h o f t...

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1 Fri. May 4, 2007 Phy107 Lecture 39 1 From Last Time… • Discussed the weak interaction • All quarks and leptons have a ‘weak charge’ – They interact through the weak interaction • Weak interaction often swamped by electromagnetic or strong interaction. • Interaction with the “weak field” by exciting an exchange boson changes one particle into another. Fri. May 4, 2007 Phy107 Lecture 39 2 Question Which of the following particles is not a ‘matter particle’ A. Electron B. Quark C. Gluon D. Neutrino Fri. May 4, 2007 Phy107 Lecture 39 3 Fundamental Matter Particles Three generations of leptons and quarks All feel the weak force. Charged leptons feel weak & EM forces Quarks feel weak, EM, & strong Fri. May 4, 2007 Phy107 Lecture 39 4 The Standard Model 6 leptons 6 quarks 4 interactions Electromagnetic, Gravitational, Strong, Weak Fri. May 4, 2007 Phy107 Lecture 39 5 Carriers of the weak force Like the Electromagnetic & Strong forces, the Weak force is also mediated by “force carriers”. A particle with a “weak charge” can couple to the “weak field” and excite one of these particles W + W - Z 0 These “weak force” carriers carry electric charge also ! This “weak force” carrier is electrically neutral Fri. May 4, 2007 Phy107 Lecture 39 6 Ice Cube Neutrino telescope, but detects light!
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2 Fri. May 4, 2007 Phy107 Lecture 39 7 Neutrino into muon •Neutrino interacts with quarks inside nucleons in nucleus. •Interaction: Neutrino emits W + , changing into muon. Down quark in neutron absorbs W + , changes to up quark. •Nucleon now has 2 ups 1 down, --> a proton! ν μ d u d n W + u u d p μ - time Fri. May 4, 2007 Phy107 Lecture 39 8 Question Which of the following particles cannot interact with a particle that carries only the weak charge? A. Quark B. Neutrino C. Electron D. All of them can Fri. May 4, 2007 Phy107 Lecture 39 9 Quark flavors • Each quark has a different "flavor." – For instance, the flavor of an up quark is simply "up.” – Up quark changing to down quark is changing flavor. – A generic way of referring to differences between quarks. • Leptons also have a ‘flavor’ – Neutrino changing into a muon is ‘changing flavor’ Fri. May 4, 2007 Phy107 Lecture 39 10 Changing flavors • Flavor change can occur spontaneously. – Experimentally, this occurs within a lepton generation e ν e Generation I Electron is stable μ ν μ Generation II Emit W - 2x10 -6 seconds τ ν τ Generation III Emit W - 3x10 -13 seconds Charge -1 0 Fri. May 4, 2007 Phy107 Lecture 39 11 “Lepton pairs” • Think of lepton pairs as a single particle with two states.
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Phy107Spr07Lect39 - Question From Last Time W h ic h o f t...

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