SM_PDF_chapter31

SM_PDF_chapter31 - Particle Physics CHAPTER OUTLINE 31.1...

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849 Particle Physics CHAPTER OUTLINE 31.1 The Fundamental Forces in Nature 31.2 Positrons and Other Antiparticles 31.3 Mesons and the Beginning of Particle Physics 31.4 Classification of Particles 31.5 Conservation Laws 31.6 Strange Particles and Strangeness 31.7 Measuring Particle Lifetimes 31.8 Finding Patterns in the Particles 31.9 Quarks 31.10 Colored Quarks 31.11 The Standard Model 31.12 Context Connection Investigatin g the Smallest System to Understand the Largest ANSWERS TO QUESTIONS Q31.1 Strong Force—Mediated by gluons. Electromagnetic Force—Mediated by photons. Weak Force—Mediated by W + , W , and Z 0 bosons. Gravitational Force—Mediated by gravitons. Q31.2 In the quark model, all hadrons are composed of smaller units called quarks. Quarks have a fractional electric charge and a baryon number of 1 3 . There are 6 types of quarks: up, down, strange, charmed, top, and bottom. Further, all baryons contain 3 quarks, and all mesons contain one quark and one anti-quark. Leptons are thought to be fundamental particles. Q31.3 Hadrons are massive particles with structure and size. There are two classes of hadron: mesons and baryons. Hadrons are composed of quarks. Hadrons interact via the strong force. Leptons are light particles with no structure or size. It is believed that leptons are fundamental particles. Leptons interact via the weak force. Q31.4 Baryons are heavy hadrons with spin 1 2 or 3 2 , are composed of three quarks, and have long lifetimes. Mesons are light hadrons with spin 0 or 1, are composed of a quark and an antiquark, and have short lifetimes. Q31.5 Resonances are hadrons. They decay into strongly interacting particles such as protons, neutrons, and pions, all of which are hadrons. Q31.6 The baryon number of a proton or neutron is one. Since baryon number is conserved, the baryon number of the kaon must be zero. Q31.7 Yes, protons interact via the weak interaction; but the strong interaction predominates. Q31.8 Decays by the weak interaction typically take 10 10 s or longer to occur. This is slow in particle physics. Q31.9 The decays of the muon, tau, charged pion, kaons, neutron, lambda, charged sigmas, xis, and omega occur by the weak interaction. All have lifetimes longer than 10 13 s. Several produce neutrinos; none produce photons. Several violate strangeness conservation.
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850 Particle Physics Q31.10 The decays of the neutral pion, eta, and neutral sigma occur by the electromagnetic interaction. These are three of the shortest lifetimes in Table 31.2. All produce photons, which are the quanta of the electromagnetic force. All conserve strangeness. Q31.11 You can think of a conservation law as a superficial regularity which we happen to notice, as a person who does not know the rules of chess might observe that one player’s two bishops are always on squares of opposite colors. Alternatively, you can think of a conservation law as identifying some stuff of which the universe is made. In classical physics one can think of both matter and energy as fundamental constituents of the world. We buy and sell both of them. In
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This note was uploaded on 10/18/2008 for the course PHYS 3Q2341234 taught by Professor Dafsf during the Spring '08 term at UCLA.

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SM_PDF_chapter31 - Particle Physics CHAPTER OUTLINE 31.1...

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