Disclaimer: this is a homework question, so I am happy with just a hint or the expressions needed to proceed with my understanding.
I am working on the momentum conservation of a particle/anti-particle annihilation process, and I have been asked to show that the annihilation of a particle with a finite mass and its anti-particle cannot lead to the emission of only one photon.
I understand why this happens: the conservation of momentum. However, I would like show this in a more sophisticated 4-momentum proof...how would I go about showing that momentum is conserved for two photons but it is not conserved when the annihilation process creates just one photon...?
This may be a duplicate of: Proving the conservation of 4-momentum for a particle collision $A+B\to C+D$