Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 301 / 3 / iv / a

The required QED Feynman rules are the electron-photon vertex $-ie{\gamma }^{\mu }$, the internal electron propagatorthe external spinors $u(p,s)$ and $\bar{u}(p^{\prime },s^{\prime })$, and photon factors ${ϵ}_{\mathrm{in}}^{\nu }$ and ${ϵ}_{\mathrm{out}}^{\ast \mu }$. Momentum conservation is imposed at both vertices.