= Unpolarized cross section
{title2=$\overline{|\mathcal M|^2}=\frac1{(2s_1+1)(2s_2+1)}\sum_{\mathrm{initial,final}}|\mathcal M|^2$}
For uncorrelated incoming beams with equally populated <spin> states, the measured <cross section> averages over initial <polarizations> and sums over unobserved final <polarizations>. The displayed average applies to two massive incoming particles with spins $s_1,s_2$. It is distinct from a prepared-polarization channel and from the final-state <identical-particle factor in a final-state phase-space integral>. A polarization channel with an amplitude that grows faster than the others can reach a perturbative bound before the averaged quantity does.
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