Suppose you have an unknown atom and you shoot an alpha particle at it in a nebula chamber. Then you measure the scattering angles of the alpha particle and the atom. You see they scatter in perpendicular directions. The problem is now to determine the mass of the unknown atom. The first thing to come to mind is to use conservation of 4-momentum. So p + q = p' + q'. But i'm wondering if i can determine the mass of the atom alone on the information of the scattering angle. In every attempt I make I end up with an equation that contains the energies of the particles, but I don't know them, because they weren't given in the problem. Only the angles. Can anyone give me a hint??