hi, I have the reaction: (photon) + (proton) ---> (neutron) + (pion+) I have to find the minimum energy of the photon. A first condition can be that total energy of photon and proton has to be at least the mass energy of the output, but I don't think this can be a satisfying solution... When I try to include quadrimomentum conservation, calculus becomes very complex and I cannot find a solution... Suggestions? Thanks.