According to wikipedia:
some grand unification theories, such as the Georgi–Glashow model, require it. According to such theories, the proton has a half-life of about 1031 to 1036 years and decays into a positron and a neutral pion that itself immediately decays into 2 gamma ray photons: $$ p^+ \to e^+ + \pi^0 $$ $$ \pi^0 \to 2 \gamma$$ Since a positron is an antilepton this decay preserves B-L number, which is conserved in most GUTs.
Can you explain why a proton is expected to decay into a positron and a pion? a neutral pion has mass 135 MeV, so we need half a dozen of them to make up the proton mass, else there should be a positron, a pion and gamma rays of 1700 GeV energy. What am I missing?
Also, a proton has 3 quarks, a pion only 2, what should happen to the missing quark? Is that hypothesis viable?