Hi, Got what seems like it should be a basic particle physics problem that I can't quite get my head around, I could do with some guidance. Here's the problem: I have electrons with 4GeV energy striking target protons and producing some neutral particle Q. I'm supposed to calculate the maximum mass of Q that could possibly be produced, and all I am told is the electron mass is 0.511MeV/c2 and the proton mass is 938.3Mev/c2. I assume these are rest masses. So, attempt at solution... I first tried thinking of things in the proton's rest frame. The proton momentum Pp1 = 0, and the electron has all the momentum in this system, Pe1. Once the electron collides with the proton, it will lose energy/momentum and Q will be produced using some of this. So, before collision, (total momentum) Ptot = Pp1 + Pe1 = Pe1 and after collision, Ptot = Pe2 + Pp2 + Pq Then I tried thinking of things in the centre of mass frame, ie. the frame where the proton and electron momenta are equal and opposite: Ptot' = Pp' + Pe' = 0, or Pe' = -Pp' Since I am looking for the most masive Q possible, I thought that all the proton and electron momenta should go into producing Q, so after the collision there would be 3 stationary particles just sitting there, ie. Ptot' = Pp2' + Pe2'+ Pq' = 0 This is where I start to get a bit confused. If I've got E2 = p2c2 + m2c4 or if c=1, then E2 = p2 + m2 then I can use these to work out the electron's initial momentum in the proton frame, but I'm not too sure where I can go from there. Do I need to somehow get what the electron's momentum (and therefore the proton's momentum) is in the centre of mass frame? This is confusing me a lot, I've been trying to do this for ages now. I appreciate it someone could break it down for me. Thanks.