In a two-particle collision (or a two-particle decay) the square of the invariant mass (in natural units) is M 2 = ( E 1 + E 2 ) 2 − − ‖ p 1 + p 2 ‖ 2 = m 1 2 + m 2 2 + 2 ( E 1 E 2 − − p 1 ⋅ ⋅ p 2 ) .