# ch41 - CHAPTER 41 Elementary Particles and the Beginning of...

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CHAPTER 41 Elementary Particles and the Beginning of the Universe 1* · How are baryons and mesons similar? How are they different? Similarities: Baryons and mesons are hadrons, i.e., they participate in strong interaction. Both are composed of quarks. Differences: 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 mass. How do these particles differ? The muon is a lepton. It is a spin- 1/2 particle; it 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; it is a boson. It does participate in strong interactions. It is composed of quarks. 3 · How can you tell whether a decay proceeds via the strong interaction or the weak interaction? A decay process involving the strong interaction has a very short lifetime (~10 -23 s); decay processes that proceed via 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 5* · True or false: Mesons are spin- 1/2 particles. False 6 · Two pions at rest annihilate according to the reaction p + + p g + g . ( a ) Why must the energies of the two g rays be equal? ( b ) Find the energy of each g ray. ( c ) Find the wavelength of each g ray. (a) The initial momentum is zero; therefore the final momentum must be zero. The momentum of a photon is E / c . To conserve both momentum and energy the two photons must have the same momentum magnitude, hence the same energy. ( b ) Use Table 41-1 ( c ) l = 1240/ E ( E in MeV, l in fm) E g = 139.6 MeV l = 8.88 fm 7 · Find the minimum energy of the photon needed for the following pair-production reactions: ( a ) g p + + p , ( b ) g p + p , and ( c ) g m + m + .

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Chapter 41 Elementary Particles and the Beginning of the Universe ( a ), ( b ), ( c ) E = 2 m i c 2 ; see Tables 41-1 and 41-3 ( a ) E = 279.2 MeV; ( b ) E = 1877 MeV; ( c ) E = 211.3 MeV 8 · State which of the decays or reactions that follow violate one or more of the conservation laws, and give the law or laws violated in each case: ( a ) p + n + e + + n e , ( b ) n p + + p , ( c ) e + + e g , ( d ) p + p g + g , and ( e ) n e + p n + e + . ( a ) Energy conservation is violated; m p < m n . ( b ) Energy conservation is violated; m n < m p + m p . ( c ) Momentum conservation is violated; two (or more) g rays must be emitted to conserve momentum. ( d ) This is an allowed process; none of the conservation laws are violated. ( e
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ch41 - CHAPTER 41 Elementary Particles and the Beginning of...

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