1. The problem statement, all variables and given/known data In a fixed target experiment a particle of mass M and kinetic energy T strikes a stationary particle of mass M. By evaluating s, t and u in the laboratory frame and using the above relation, or otherwise, show that the kinetic energy T' of the particle scattered elastically at an angle θ is given by T' = T cos^2 θ / (1 + ((T/2M)*sin^2 θ)) 2. Relevant equations We know that: We define P1 as the four momentum of the incoming proton: P1 = (T+M,0,0,T) P2 as the stationary proton: P2 = (M,0,0,0) P3 as the initially incoming proton after the scattering event: P3 = (T'+M, 0, T'sinθ, T'cosθ) P4 as the initially stationary proton after the scattering event: P4 = (E4+M, P4) where P4 in the bracket is a 3 vector not 4. We also know: s = (P1 + P2)2 , t = (P1 − P3)2 and u = (P1 − P4)2 and s + t + u = ∑ mi2 = 4m2 3. The attempt at a solution I have attempted this multiple times over a few days, I realise that P4 and therefore u need to be eliminated as they aren't in the solution but I am unsure how. Any help/ suggestions would be appreciated. Thanks, John.