Center-of-mass energy
= Center-of-mass energy
{title2=$E_{\rm com}^2=-P_\mu P^\mu$}
= Centre-of-mass energy
{synonym}
For future-directed particles with total <four-momentum> $P$ and <metric signature> $(-+++)$, the positive square root of $-P_\mu P^\mu$ is their energy in the frame where total spatial momentum vanishes. Its invariant definition remains valid at a collision in <spacetime>.