A process of the type

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 much longer lifetime. 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.

The whole issue of proton decay now looks much more complicated than it did in the early days of GUTs in the mid-1970s. It has been suggested that some discrete group may be important in stabilizing protons against rapid decay.