At this moment, Tom still didn't know the specific decay path of proton decay.
It might decay into a positron and a pion. If so, the pion would then decay into two photons, and the positron would annihilate with surrounding electrons, also producing additional photons.
It could also decay via another path, into a muon and a kaon. And the kaon would also decay into a pion, which would subsequently decay into photons.
Or it could decay into a neutrino and a kaon.
The different decay paths Tom speculated on shared a commonality: the final products always included photons.
