= Invariant scattering amplitude
{title2=$\langle f|S-1|i\rangle=i(2\pi)^4\delta^{(4)}(P_f-P_i)\mathcal M$}
With relativistically normalized external states, $\mathcal M$ is the reduced <S-matrix> element after removing the <four-momentum conservation> distribution. A <relativistic scattering cross-section> combines $|\mathcal M|^2$ with the <Lorentz-invariant phase-space measure> and the <invariant flux factor>. Identical-leg factorials in <Feynman vertices> and the <identical final-state symmetry factor> in the event integral have different roles.
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