I have been introduced to Feynman diagrams in a particle physics University module and I just created this diagram to practice checking conservation laws:
I have been told that charge, baryon and lepton numbers have to be conserved at vertices, and this is true for my diagram. I have also been told that "if the exchange particle is a neutral boson (photon, gluon, Z), quark and lepton flavours have to be conserved". Pions are also bosons, since their spin is zero (integer). So since the lepton flavour is changed in this diagram, that means that this interaction would not be allowed. However, assuming it was allowed, I then ask myself this question: which type of interaction would be going on here? It is not the strong force since leptons are not colour charged, and it is not electromagnetic since this pion is neutral. So I am left with weak nuclear force. Then would this diagram be part of a more complicated one involving W or Z boson exchanges? If someone could clarify my thoughts I would be so grateful.