A process of the type
p → e+ + π0
where a proton decays into a positron and a pion, predicted to occur in grand unified theories (GUTs) because baryon number is no longer conserved. The lifetime depends on the theory used and is typically 1035 years, but a combination of GUTs and supersymmetry gives a lifetime of about 1045 years. Considerable experimental effort has been spent in looking for proton decay, so far with no success. This negative result means that GUTs that are not combined with supersymmetry can be ruled out.