Møller scattering

Elastic electron-electron scattering. The electromagnetic tree-level Feynman diagrams exchange a photon in the $t$ and $u$ channels, with a relative minus sign from the identical fermions. In the massless limit the initial-spin-averaged squared scattering amplitude is $2e^4[(s^2+u^2)/t^2+(s^2+t^2)/u^2+2s^2/(tu)]$. The exchange interference is essential. A final-state counting factor for integrating a cross section is separate from this squared scattering amplitude.