Relativistic scattering cross-section
= Relativistic scattering cross-section
With relativistically normalized external states, a two-particle initial state has differential cross-section $d\sigma=|\mathcal M|^2d\Phi_n/\mathcal F$, where $d\Phi_n$ is the <Lorentz-invariant phase-space measure> and $\mathcal F$ is the <invariant flux factor>. For identical unobserved final particles, divide the full labeled phase-space integral by their permutation multiplicity.