 Solution

ID: past-exam-of-the-particle-and-nuclear-physics-course-of-the-university-of-cambridge/2016/paper-1/3/a/ii/solution

This is a weak charged-current decay. Panel (ii) has one

W^{+} e^{+} ν_{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.

 New to topics? Read the docs here!