proton decay

In particle physics, proton decay is a hypothetical form of radioactive decay in which the proton decays into lighter subatomic particles, usually a neutral pion and a positron. Proton decay has not been observed.In the Standard Model, protons, a type of baryon, are theoretically stable because baryon number is approximately conserved. That is, they will not decay perturbatively into other particles on their own because they are the lightest (and therefore least energetic) baryon.Beyond-the-Standard Model grand unified theories (GUTs) explicitly break the baryon number symmetry, allowing protons to decay via new X bosons. Proton decay is one of the few observable effects of the various proposed GUTs. To date, all attempts to observe these events have failed.
See more at Wikipedia.org...