1. The problem statement, all variables and given/known data Write the invariant s = (P1+P2)2 as a function of masses amd energies of the process 1+2 → 3+4 in the center of momentum frame and on the lab frame, in which b is at rest. Interpret the result. 2. Relevant equations 3. The attempt at a solution For the CoM frame I have: s = (P1+P2)2 = (E1+E2, p1 + p2)2 = (E1+E2)2 (and similar for particles 3 and 4) In the lab frame (primed symbols are used for variables in this frame) s' = (P'1+P'2)2 = (E'1+E'2, p'1)2 = (E'1+E'2)2 + |p1|2 = (E'1+E'2)2 + m012-E'12 = 2E'1E'2+m012+m022 (and similar for particles 3 and 4) Is this right? I still don't feel secure with this problems .Is if is right, can we get any interesting conclusions from the result? We must have s = s', so 2E'1E'2+m012+m022 = 2E1E2+E12+E22 And I get to E'1E'2 = E1E2 + |p1|2. If the energies were being summed I would interpret it as the fact that the energies in the lab frame is the sum of the energies in the CoM frame plus the momentum of the particles, or something like that, but this means nothing to me.