Canonical Quantization of Scalar Fields — Mark Srednicki
Physics — QFT · moller-scattering
Problem
Compute ⟨∣T∣2⟩ for M\o ller scattering, e−e−→e−e−. You should find that your result is the same as that for e+e−→e+e−, but with s↔u. This is another example of crossing symmetry.