Lorentz-invariant phase space
= Lorentz-invariant phase space
{c}
{title2=$d\Phi_2$}
For two outgoing particles the <Lorentz-invariant phase space> is $d\Phi_2=(2\pi)^4\delta^{(4)}(p-k-k')\,d^3k/((2\pi)^32E_k)\,d^3k'/((2\pi)^32E_{k'})$. With massless daughters in their center-of-mass frame, $d\Phi_2=d\Omega/(32\pi^2)$ and $\int d\Phi_2=1/(8\pi)$. This fixes normalization factors in <decay widths> and scattering rates.