1. The problem statement, all variables and given/known data Consider two photons, one with energy ε1 = 2MeV traveling to the right, and the other with energy ε2 = 3MeV moving tot he left. The two photons collide head-on and produce a positron-electron pair. Suppose the the electron and positron move along the same axis as the photons. What are the final energies (Ee- and Ee+) and velocities (ve- and ve+) of the positron and the electron? (Hint: it is easier to do this problem by first switching to a frame of reference where the two photons have the same energy (and thus same momentum); in this frame, after the collision the center of mass is at rest.) 2. Relevant equations ε1 = 2 MeV ε2 = 3 MeV me = 0.511 MeV/c2 p = ϒmv E = ϒmc2 = mc2 + EK = mc2 + mc2(ϒ-1) E = √[(pc)2 + (mc2)2] ϒ = 1/√1-(v2/c2) 3. The attempt at a solution I set up four frames of reference (FORs): E: lab frame, before collision E' : lab frame, after collision Ecp: Center-of -mass frame before collision E'cp: Center-of -mass frame after collision E = c(p1 + p2) = 5MeV E'cp = 2mec2 E' = 2mec2 + EK I know that the norms of the energies will be equal from one FOR to another, so: E2 = (E'cp)2 (E'cp)2 = 25 MeV2 And this is where I'm stuck. We've covered four-vectors, and I think I might be getting confused on whether or not I use them here, and if so, how to set up the components for the photons.