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

Multiplying the QED Feynman rules in fermion-line order gives, up to the common convention for $i\mathcal{M}$,The two terms are both required by the Ward identity; replacing either external polarization by its photon momentum makes their sum vanish after using momentum conservation and the external Dirac equations.