SM_chapter46

SM_chapter46 - 46 Particle Physics and Cosmology CHAPTER...

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46 Particle Physics and Cosmology 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 Classi± cation of Particles 46.5 Conservation Laws 46.6 Strange Particles and Strangeness 46.7 Finding Patterns in the Particles 46.8 Quarks 46.9 Multicolored Quarks 46.10 The Standard Model 46.11 The Cosmic Connection 46.12 Problems and Perspectives 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 Answer (b). The electron and positron together have very little momentum. A 1.02-MeV photon has a def nite chunk o± momentum. Production o± a single gamma ray could not satis±y the law o± conservation o± momentum, which must hold true in this—and every—interaction. Q46.3 Hadrons are massive particles with structure and size. There are two classes o± hadrons: mesons and baryons. Hadrons are composed o± quarks. Hadrons interact via the strong ±orce. Leptons are light particles with no structure or size. It is believed that leptons are ±unda- mental particles. Leptons interact via the weak ±orce. Q46.4 Baryons are heavy hadrons with spin 1 2 or 3 2 composed o± three quarks. Mesons are light hadrons with spin 0 or 1 composed o± a quark and an antiquark. *Q46.5 Answer (c). The muon has much more rest energy than the electron and the neutrinos together. The missing rest energy goes into kinetic energy. *Q46.6 Answer (b). The z component o± its angular momentum must be 3/2, 1/2, –1/2, or –3/2, in units o± h . Q46.7 The baryon number o± a proton or neutron is one. Since baryon number is conserved, the baryon number o± 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. *Q46.9 Answer a, b, c, and d. Protons ±eel all these ±orces; but within a nucleus the strong interaction predominates, ±ollowed by the electromagnetic interaction. 591
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Q46.10 You can think of a conservation law as a superF cial 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 classical physics you can also think of linear momentum, angular momentum, and electric charge as basic stuffs of which the universe is made. In relativity we learn that matter and energy are not conserved separately, but are both aspects of the conserved quantity relativistic total energy .
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This note was uploaded on 01/19/2012 for the course EE 421 taught by Professor Zhicheng during the Spring '11 term at Ohio State.

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SM_chapter46 - 46 Particle Physics and Cosmology CHAPTER...

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