\b[This is a weak charged-current decay.] Panel (ii) has one $W^+e^+\nu_e$ vertex. The outgoing <Positron> and <electron neutrino> have total charge $+1$ and cancelling <lepton numbers>. The <W boson> is on shell here; a second virtual boson is unnecessary.

Solution

Past exam of the particle and nuclear physics course of the University of Cambridge / 2016 / Paper 1 / 3 / a / ii / Solution

 Solution (source code)