A photon hits a proton that is at rest (In the lab system) which creates a pion, so now there are only the proton and pion. The question is what minimal energy can you give a photon such that this reaction is possible?
I tried using conservation of energy and momentum however the $\gamma$ in the equations made it impossible to solve. I also tried moving to the c.m. system but because that changes the frequency and therefore the energy of the photon the algebra is again to complicated.