= Two-body scattering cross section
{title2=$\frac{d\sigma}{d\Omega}=\frac1{64\pi^2s}\frac{|\mathbf k_f|}{|\mathbf p_i|}\overline{|\mathcal M|^2}$}
For distinguishable final particles, the <centre-of-momentum frame> cross section is the displayed product of the invariant amplitude, incoming flux and <relativistic two-body phase space>. Initial unpolarized states are averaged; unobserved final polarizations are summed. An isotropic amplitude integrates to $\sigma=(|\mathbf k_f|/|\mathbf p_i|)\overline{|\mathcal M|^2}/(16\pi s)$. A full-solid-angle integral for identical final particles additionally needs its symmetry factor.
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