Pretty simple; what are some different ways that photons are released? Specifically, I'm looking for a short list of different particles that will release a photon in any given interaction. Tried googling, but its amazing how difficult it is to find an answer to such a simple question -- basically, someone is telling me that photons are released exclusively by electrons, but I'm 99% sure that isn't true and just want an external source other than my own mouth to be able to correct them (the first thing that pops into my head is a proton and antiproton annihilating each other into pure energy - two high frequency photons). Who knows, I might be wrong though, and could just be entirely deluded with what I thought I knew.. But I think since 'how an electron produces a photon' is one of the first examples a textbook/teacher gives when teaching electromagnetism and the photoelectric effect, so it'd be easy to make the mistake of thinking its the only way a photon is released especially if no more examples are given. Also, I've got another question if anyone can give me a quick solid answer: When two protons (just two protons, aka two positive +1 hydrogen ions) get in close proximity to each other and repel each other since they both have a positive charge, is this repulsion mediated by a photon or just the electromagnetic field of the protons themselves?