1. The problem statement, all variables and given/known data An electron of energy 10GeV strikes a proton at rest. What is the velocity of the electron-proton CM system? What energy is available to produce new particles? 2. Relevant equations 3. The attempt at a solution I've done the first part, but am stuck on the second. I was thinking that in the CM frame after the collision, as the sum of momenta is zero, if the max amount of energy possible went into new particles, ECM = mc^2 + Mc^2 + E* where m is the mass of an electron, M the mass of a proton and E* the energy going into new particles. I have ECM from the first part, so I can get E*. However I don't know how to transform this energy into energy in the lab frame. I (hopefully) won't need a full solution, just some pointers on how to approach this please!!