Two-body Lorentz-invariant phase space
= Two-body Lorentz-invariant phase space
{title2=$d\Phi_2=d\Omega/(32\pi^2)$}
For two distinct massless final particles in their centre-of-momentum frame, integrating the energy-momentum delta function gives $d\Phi_2=d\Omega/(32\pi^2)$. A massless two-particle initial state has flux $2s$, hence $d\sigma/d\cos\theta=\overline{|\mathcal M|^2}/(32\pi s)$. Average only over the initial spin states physically present.