The term invariant mass is also used in inelastic scattering experiments. Given an inelastic reaction with total incoming energy larger than the total detected energy (i.e. not all outgoing particles are detected in the experiment), the invariant mass (also known as the "missing mass") W of the reaction is defined as follows (in natural units): W 2 = ( ∑ ∑ E in − − ∑ ∑ E out ) 2 − − ‖ ∑ ∑ p in − − ∑ ∑ p out ‖ 2 .