Relativistic two-body phase space ($\frac{d{\Phi }_2}{d\Omega }=\frac{k}{16{\pi }^2\sqrt{s}}$)

For two outgoing particles with centre-of-mass momentum magnitude $k$ and total energy $\sqrt{s}$, integrating the four-momentum delta function gives $d{\Phi }_2/d\Omega =k/(16{\pi }^2\sqrt{s})$. A factor $1/2!$ for identical outgoing particles is inserted when counting events over the full phase space, not into this labelled kinematic measure itself.