Chapter 41

# Chapter 41 - CHAPTER 41 Elementary Particles and the...

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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 π + + γ + . ( a ) Why must the energies of the two rays be equal? ( b ) Find the energy of each ray. ( c ) Find the wavelength of each 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 ) λ = 1240/ E ( E in MeV, in fm) E = 139.6 MeV = 8.88 fm 7 Find the minimum energy of the photon needed for the following pair-production reactions:

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Chapter 41 Elementary Particles and the Beginning of the Universe ( a ) γ π + + , ( b ) p + p , and ( c ) µ + + . ( 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 + + ν e , ( b ) n p + + , ( c ) e + + e , ( d ) p + p + , and ( e ) e + p n + e + . ( a ) Energy conservation is violated; m p < m n . ( b ) Energy conservation is violated; m n < m p + m . ( c ) Momentum conservation is violated; two (or more) rays must be emitted to conserve momentum. ( d ) This is an allowed process; none of the conservation laws are violated. ( e ) This is an allowed process; the lepton number is -1 on both sides and energy is conserved. 9* Determine the change in strangeness in each reaction that follows, and state whether the reaction can proceed via the strong interaction, the weak interaction, or not at all: ( a ) Ξ 0 + , ( b ) Ξ 0 p + + 0 , and ( c ) Λ 0 p + + .
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## Chapter 41 - CHAPTER 41 Elementary Particles and the...

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