Past exam of the mathematics course of the University of Cambridge / 2023 / iii / Paper 301 / 4 / ii / Solution

LabelThere are two electron-exchange diagrams, distinguished by which labelled outgoing photon is emitted first along the fermion line. They are the $t$- and $u$-channel topologies. Their sum isThe two terms are added because the external photons are identical bosons. Replacing either polarization by its momentum makes their sum vanish by the Ward identity.