1. The problem statement, all variables and given/known data At HERA 30 GeV electrons collided head on with 820 GeV protons. Calculate the invariant mass of ep collisions. (masses: e=0.0005GeV, p=0.938GeV) 2. Relevant equations M^2 = (E1 + E2)^2 - (p1 + p2)^2 ? 3. The attempt at a solution I know the numerical answer to this question is 314GeV, but so far I haven't been able to get it. I've tried plugging in the known values to the above equation, using E^2 = p^2 + m^2 to find momentum, but obviously didn't get the right answer. What should I be doing? Thanks!