This question was brought to mind while reading other threads. I didn't think it was appropriate to diverge those threads off their subjects, so I started a new one. (hope this is right). I'm specifically thinking about the extremely rare event of the interaction of two gamma rays as the pair production event. It would seem that there should be an intermediate 'combined' particle since the event can't be instantaneous. Does this particle have a name? What would its properties have to be? (Is it spin 2 like a graviton?) This is not my field, just interested. Thank you.