Lorentz-invariant phase-space measure
= Lorentz-invariant phase-space measure
{c}
{title2=$d\Phi_n(P)=(2\pi)^4\delta^4(P-\sum_i p_i)\prod_i\frac{d^3p_i}{(2\pi)^3 2E_i}$}
The on-shell measure $d^3p/(2E)$ is Lorentz invariant. The product measure together with the four-dimensional <Dirac delta distribution> enforces <four-momentum conservation>. For equal masses in two-body scattering, $d\Phi_2=\sqrt{1-4m^2/s}\,d\Omega/(32\pi^2)$.