Chap 41 SM - Chapter 41 Elementary Particles and the...

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Chapter 41 Elementary Particles and the Beginning of the Universe Conceptual Problems 1 How are baryons and mesons similar? How are they different? Similarities Differences Baryons and mesons are hadrons, i.e., they participate in the strong interaction. Both are composed of quarks. Baryons consist of three quarks and are fermions. Mesons consist of two quarks and are bosons. Baryons have baryon number +1 or 1. Mesons have baryon number 0. 2 The muon and the pion have nearly the same masses. How do these particles differ? Determine the Concept The muon is a lepton. It is a spin-½ particle and is a fermion. It does not participate in strong interactions. It appears to be an elementary particle like the electron. The pion is a meson. Its spin is 0 and it is a boson. It does participate in strong interactions and is composed of quarks. 3 [SSM] How can you tell whether a decay proceeds by the strong interaction or the weak interaction? Determine the Concept A decay process involving the strong interaction has a very short lifetime ( 10 23 s), whereas decay processes that proceed by the weak interaction have lifetimes of order 10 10 s. 4 True or false: ( a ) All baryons are hadrons. ( b ) All hadrons are baryons. ( a ) True ( b ) False. There are two kinds of hadrons-baryons, which have spin 2 1 (or 2 3 , 2 5 and so on), and mesons, which have zero or integral spin. 5 True or false: All mesons are spin- 1 2 particles. False. Mesons have zero or integral spins. 363

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Chapter 41 364 6 A particle made of exactly two quarks is ( a ) a meson, ( b ) a baryon, ( c ) a lepton, ( d ) either a meson or a baryon, but definitely not a lepton? Determine the Concept A meson has 2 quarks, a baryon has 3 quarks. Hence () a is correct. 7 Have any quark–antiquark combinations whose electric charge is not an integral multiplied by the fundamental charge e been observed? Determine the Concept No; from Table 41-2 it is evident that any quark- antiquark combination always results in an integral or zero charge. 8 True or false: ( a ) A lepton is a combination of three quarks. ( b ) The typical times for decays by the weak interaction are orders of magnitude longer than the typical times for decays by the strong interaction. ( c ) The muon and the pion are both mesons. ( a ) False. Leptons are not made up of quarks. ( b ) True ( c ) False. The muon is lepton, not a meson. 9 True or false: ( a ) Electrons interact with protons by the strong interaction. ( b ) Strangeness is not conserved in reactions involving the weak interaction. ( c ) Neutrons have zero charm. ( a ) False. Electrons interact with protons by the electromagnetic interaction. ( b ) True ( c ) True Estimation and Approximation 10 •• Grand unification theories predict that the proton has a long but finite lifetime. Current experiments based on detecting the decay of protons in water infer that this lifetime is at least 10 32 years. Assume 10 32 years is, in fact, the mean lifetime of the proton. Estimate the expected time between proton-decays
Elementary Particles and the Beginning of the Universe 365 that occur in the water of a filled Olympic-size swimming pool. An Olympic-size

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This note was uploaded on 02/28/2012 for the course PHYSICS 122 taught by Professor Goussiou during the Spring '12 term at University of Washington.

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Chap 41 SM - Chapter 41 Elementary Particles and the...

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