Q99 · Practice questionComplex unfamiliar5 marks
QUESTION 99 (5 marks)
At quark level, beta-minus decay can be represented by a down quark changing into an up quark and emitting a boson, which then produces an electron and an electron antineutrino. Explain how this converts a neutron into a proton and verify conservation of electric charge and lepton number.
WORKED SOLUTION
5 marksPractice marking scheme
ANSWER
via the weak interaction; charge and lepton number are conserved.
Worked solution
A neutron has quark content . If one quark changes to a quark, the baryon becomes , which is a proton. At the quark vertex, charge changes from to , a difference of , so a of charge is emitted, conserving charge. The then decays to : charge is . Lepton number is initially 0 for the hadronic system; the final electron has and the electron antineutrino has , so total lepton number remains 0.
Links d to u conversion with neutron to proton.
Identifies weak interaction/W boson role.
Verifies charge at quark vertex.
Verifies charge in W decay.
Verifies lepton-number conservation.
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