Chapter 46 - 46 Particle Physics and Cosmology CHAPTER...

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46 CHAPTER OUTLINE 46.1 The Fundamental Forces in Nature 46.2 Positrons and Other Antiparticles 46.3 Mesons and the Beginning of Particle Physics 46.4 Classification of Particles 46.5 Conservation Laws 46.6 Strange Particles and Strangeness 46.7 Making Elementary Particles and Measuring Their 46.8 Finding Patterns in the Properties Particles 46.9 Quarks 46.10 Multicolored Quarks 46.12 The Cosmic Connection 46.11 The Standard Model Problems and Perspectives 46.13 Particle Physics and Cosmology ANSWERS TO QUESTIONS Q46.1 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. Q46.2 The production of a single gamma ray could not satisfy the law of conservation of momentum, which must hold true in this—and every—interaction. Q46.3 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. Q46.4 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. Q46.5 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. Q46.6 Resonances are hadrons. They decay into strongly interacting particles such as protons, neutrons, and pions, all of which are hadrons. Q46.7 The baryon number of a proton or neutron is one. Since baryon number is conserved, the baryon number of the kaon must be zero. Q46.8 Decays by the weak interaction typically take 10 10 s or longer to occur. This is slow in particle physics. 619
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620 Particle Physics and Cosmology Q46.9 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. Q46.10 The decays of the neutral pion, eta, and neutral sigma occur by the electromagnetic interaction. These are three of the shortest lifetimes in Table 46.2. All produce photons, which are the quanta of the electromagnetic force. All conserve strangeness. Q46.11 Yes, protons interact via the weak interaction; but the strong interaction predominates. Q46.12 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
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Chapter 46 - 46 Particle Physics and Cosmology CHAPTER...

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