We have a collision involving a Kaon plus and proton initially resulting in the same plus a neutral pion (ie. Kp to Kp(pi)). The question asks to calculate the invariant mass of just the outgoing kaon and pion, given the outgoing momenta of the particles, the angle between them and their masses. Do I have to take into account the mass of the proton when I'm calculating this, or can I simply add (E1 + E2)^2 and (p1 + p2)^2 (ie the masses and momentum of the two relevant particles) according to the invariant mass equation? The value I'm getting now seems too large, in the region of 10 GeV/c^2. Any help/suggestions would be appreciated. Cheers, W.