= Tree-level Compton amplitude
For a charged Dirac particle, incoming momenta $p,k$ and outgoing momenta $p',k'$ obey $p+k=p'+k'$. The two <QED> diagrams have internal <fermion> momenta $p+k$ and $p-k'$, giving $s$ and $u$ channels. The amplitude is proportional to $\bar u(p')[\not\epsilon'^*R(p+k)\not\epsilon+\not\epsilon R(p-k')\not\epsilon'^*]u(p)$, where $R(q)=(\not q+m)/(q^2-m^2+i0)$. It is related to particle-<antiparticle> two-<photon> annihilation by <crossing symmetry>, with appropriate changes to external wavefunctions and conjugations.
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