 Muon-antimuon annihilation into two photons

At tree level this QED process has two internal muon exchange diagrams, differing by the order of the two outgoing photon vertices. Their propagator momenta are $p_1-k_1$ and $p_1-k_2$ for incoming muon momentum $p_1$. The amplitude begins at order $e^2$ and obeys the two-photon fermion Ward identity. Identical final photons require the usual factorial correction when integration over the full final-state Lorentz-invariant phase space would otherwise count them twice.