Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 305 / 3 / a / ii / Solution

Allowed. A single $t$-channel W boson mediates the weak charged current transitions $u\to d$ and $\bar{c}\to \bar{s}$. The amplitude is proportional to the Cabibbo-Kobayashi-Maskawa matrix product $V_{ud}{V}_{cs}^{\ast }$.