= Identical-particle factor in a final-state phase-space integral
= Identical final-state symmetry factor
{synonym}
When the full <Lorentz-invariant phase-space measure> treats $n$ identical final particles as labeled, each physical final configuration is counted $n!$ times. The unlabelled <relativistic scattering cross-section> therefore includes $1/n!$. For two identical scalars, this divides the full-angle result by two; equivalently, integrate over a region containing only one representative of each exchanged pair.
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