Helicity amplitude
= Helicity amplitude
{title2=$\mathcal M_{h_3h_4;h_1h_2}$}
A <scattering amplitude> evaluated with definite external <helicities>. Rotational covariance relates its angular dependence to a <Wigner D-matrix>. Summing final <helicities> and averaging an unpolarised initial mixture yields a spin-averaged squared <matrix element>.