= Electron-positron annihilation into two photons
{title2=$e^-e^+\to\gamma\gamma$}
The leading <scattering amplitude> is the sum of two <tree-level Feynman diagrams>, with the outgoing photons attached in either order to the fermion line. The <Dirac propagator> carries momentum $p-k_1$ or $p-k_2$ for incoming electron momentum $p$. Their sum obeys the <two-photon fermion Ward identity>, although individual diagrams need not. <Crossing symmetry> relates this amplitude to <Compton scattering>.
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