= Relativistic collision mass threshold
{title2=$W\geq c^2\sum_i\mu_i$}
If the total incoming <four-momentum> is timelike and has <centre-of-momentum energy> $W$, production of particles with <rest masses> $\mu_i$ requires $W\geq\sum_i\mu_ic^2$. In the <centre-of-momentum frame>, total final energy is a sum of $\sqrt{\mu_i^2c^4+c^2|\mathbf p_i|^2}$, each at least its rest energy. For massive products equality occurs with all at rest in that frame. A single product requires equality of the entire invariant mass, not just an inequality; a massless product cannot have nonzero energy and be at rest.
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