Thank you mfb. I think you gave me the right clue. Annihilation is the only outcome that satisfy conservation. One could think that other outcomes are possible. I can see that one new particle of mass 2m cannot be created because such a particle does not exist. I am not so sure about a bound...
I know that matter and corresponding antimatter particles annihilate giving energy and, conversely a photon can give a pair of particle / antiparticle.
Perhaps this is a candid question, but is it only an empiric finding? or is there other reason? In fact the two particles of the pair have...