Since the invariant mass is determined from quantities which are conserved during a decay, the invariant mass calculated using the energy and momentum of the decay products of a single particle is equal to the mass of the particle that decayed. The mass of a system of particles can be calculated from the general formula: ( W c 2 ) 2 = ( ∑ ∑ E ) 2 − − ‖ ∑ ∑ p c ‖ 2 , where