= 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.
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