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. Emitting or absorbing W toggles it between the states.
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