Electron-positron annihilation into a muon pair
= Electron-positron annihilation into a muon pair
{title2=$e^-e^+\to\mu^-\mu^+$}
At tree level, the <Feynman diagram> has an electron-positron current and a muon-antimuon current joined by one virtual <photon>. In the massless limit, the <spin average> and final-state <spin sum> give $\overline{|\mathcal M|^2}=e^4(1+\cos^2\theta)$, and the integrated <relativistic scattering cross-section> is $4\pi\alpha^2/(3s)$.