Hello everyone, Is this reaction allowed? π0 + π+ -> π+ + γ (just a single photon) I mean the π+ contributes with the reaction not just by making it possible that two particles decay into a single photon (by changing it's momentum), but also the π0 reacts with the π+, where the particle-antiparticle pairs in both mesons annihilate themselves. If this reaction is allowed, how would the feynman diagram of it look like? In this case we can't make the neutral pion look like a circle or triangle, because it has to be a specific state, not the uubar+ddbar/√2 one. The interaction has to force the π0 to be uubar or ddbar.