This raises a question in my mind that I can't answer and I wonder if anybody knowledgeable may be able to help me with. What is the QM process of annihilation between an electron and a positron and how does this give rise to EM radiation? I was thinking that we all learnt at university about transition dipole moment when an electron changes state in an atom, how this is associated with emission or absorption of a photon, how the symmetry of the process results in selection rules for transitions, and so on. What kind of interaction between the wavefunction of an electron and a positron occurs and how does this lead to generation of a photon? Presumably there must be some sort of transition dipole moment produced (related to the opposite charges I presume), a symmetry change and an angular momentum change, which are satisfied by emission of a photon. I had a quick look on the web but could not find anything obvious on the subject. Can anyone point me to a good source? I'm not looking for walls of Greek, just a short summary.