= Massless two-body scattering flux normalization
{title2=$d\sigma/d\Omega=\overline{|\mathcal M|^2}/(64\pi^2s)$}
For two distinct massless final particles, $d\Phi_2/d\Omega=1/(32\pi^2)$ in the centre-of-mass frame. The invariant initial flux is $4p_1\cdot p_2=2s$. Their ratio gives the displayed differential cross-section. Equivalently the denominator is $128\pi^2p_1\cdot p_2$. Initial spins are averaged and final spins and colors summed when defining an inclusive unpolarized cross-section; averaging final colors would change the observable.
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