I'm not going to write the full question as its not neccessary, but this is the gist of it.
Electrons are fired at protons and a search is made for a neutral particle created in the impact. Both the electron and proton still exist after the impact, along with this new particle. What is the maximum mass that can be produced. Use any reasonable approximations, but justify your arguments.
Basically it strikes me that the maximum mass of this new particle would be in the case that both the electron and proton are stationary after the collision. But is this a physically possible scenario??