This isn't a homework question, though it kind of relates to a practical I did recently. Sorry if it's posted in the wrong section! So, why are only 2 photons formed from positron-electron annihilation at rest? I understand why you can't have just one, as then you won't get conservation of momentum. But why do you only get 2 photons instead of say, 10 or 1000? I've looked up various particle physics textbooks and one on radiation detection, but they only explain that you can't get a single photon.