Q130 · Practice questionComplex unfamiliar11 marks
QUESTION 130 (11 marks)
A proton in a nucleus undergoes -plus decay, represented at the nucleon level by nu.
a)[3 marks]
Describe the quark-level change that converts the proton into a neutron.
b)[4 marks]
Verify conservation of electric charge, baryon number and electron-lepton number.
c)[4 marks]
Describe the weak-interaction vertices and identify the exchanged gauge boson.
WORKED SOLUTION
11 marksPractice marking scheme
ANSWER
At the quark vertex . The virtual carries charge and subsequently produces nu. This is a weak interaction mediated by a charged boson.
Worked solution
Part (a): A proton has valence quarks uud and a neutron udd. One up quark changes to a down quark; the other two quarks are spectators.
Part (b): Charge: . Baryon number: . Electron-lepton number: ( for ) + ( for nu). All are conserved.
Part (c): At the quark vertex . The virtual carries charge and subsequently produces nu. This is a weak interaction mediated by a charged boson.
Part (a): completes the requested reasoning and obtains a correct result.
Part (b): completes the requested reasoning and obtains a correct result.
Part (c): completes the requested reasoning and obtains a correct result.
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