Hi everyone! In an exam on particle physics there has been a problem where for a number of decays we were asked to either reason their non-existence (i.e. name a conservation law that it contradicts to) or draw a Feynman diagram. However, with one of those decays I have a problem: [itex]\gamma + p \to \pi^0 + p[/itex] Could someone please tell me the right answer? I know it is not a good style to ask for a complete solution. Of course I would also be happy with a clue or a reference, but I don't see in what way it would be possible to give a clue here without telling the result. The problem is: I don't see any conservation law that it would contradict to: Lepton numbers (there are no leptons involved), quark flavor (is respected, because pi0 is up-antiup or down-antidown), electric charge (respected) are the only ones that can be checked without a concrete Feynman diagram. On the other hand, I cannot imagine how a Feynman diagram should look like. What the hell shall I do with the photon? There is another conservation law here, 'Fermion current' or 'fermion flow' (I am not sure about the English term; the German term was "Fermionenstrom"), of that I don't really know what it means. Perhaps, is this the solution? Maybe there is a simple principle that I don't understand yet. The lecture was just a very superficial introductory lecture, no QFT.