Canonical Quantization of Scalar Fields — Mark Srednicki
Physics — QFT · compton-scattering
Problem
Compute ⟨∣T∣2⟩ for Compton scattering, e−γ→e−γ. You should find that your result is the same as that for e+e−→γγ, but with s↔t, and an extra overall minus sign. This is an example of crossing symmetry; there is an overall minus sign for each fermion that is moved from the initial to the final state.