Invariant mass

Summary

The invariant mass of a particle or of a system of particles is the Minkowski length of its total four-momentum, divided by $c$. It has the same value in every inertial frame. For a single particle it is the rest mass. For a system it is the total energy in the frame where the total momentum is zero, divided by $c^2$. In particle physics it is used to identify a decayed particle from the energies and momenta of its decay products.