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

There are two tree-level Feynman diagrams for Compton scattering: an $s$-channel ordering in which the electron first absorbs the incoming photon, with internal momentum $p+q$, and a crossed $u$-channel ordering in which it first emits the outgoing photon, with internal momentum $p-q^{\prime }$.