In the Standard Model, protons, a type of baryon, are theoretically stable because baryon number is approximately conserved. That is, they will not decay into other particles on their own because they are the lightest (and therefore least energetic) baryon.
Some 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.
Most grand unified theories (GUTs) explicitly break the baryon number symmetry, which would account for this discrepancy, typically invoking reactions mediated by very massive X bosons (X below) or massive Higgs bosons (T). The rate that these events occur is governed largely by the mass of the intermediate X or T particles, so by assuming these reactions are responsible for the majority of the baryon number seen today, a maximum mass can be calculated, above which the rate would be too slow to explain the presence of matter today. These estimates predict that a large volume of material will periodically exhibit spontaneous proton decay even given the much reduced energies available today.
Proton decay is one of the few observable effects of the various proposed GUTs, the other major one being magnetic monopoles. Both became the focus of major experimental physics efforts starting in the early 1980s. Proton decay was, for a time, an extremely exciting area of experimental physics research. To date, all attempts to observe these events have failed. Recent experiments at the Super-Kamiokande water Cherenkov radiation detector in Japan indicate that if protons decay at all, their half-life must be at least 1035 years.
Despite the lack of observational evidence for proton decay, some grand unification theories require it. According to some such theories, the proton has a half-life of about 1036 years, and decays into a positron and a neutral pion that itself immediately decays into 2 gamma ray photons:
Additional decay modes are available, both directly and when catalyzed via interaction with GUT-predicted magnetic monopoles. Though this process has not been observed experimentally, it is within the realm of experimental testability for future planned very large-scale detectors on the megaton scale. Such detectors include the Hyper-Kamiokande.
Early grand unification theories, which were the first consistent theories to suggest proton decay postulated that the proton's half-life would be at least 1031 years. As further experiments and calculations were performed in the 1990s, it became clear that the proton half-life could not lie below 1032 years. Many books from that period refer to this figure for the possible decay time for baryonic matter.
Although the phenomenon is referred to as "proton decay", the effect would also be seen in neutrons bound inside atomic nuclei. Free neutrons—those not inside an atomic nucleus—are already known to decay into protons (and an electron and an anti-neutrino) in a process called beta decay. Free neutrons have a half life of 15.4 minutes due to the weak interaction. Neutrons bound inside a nucleus have an immensely longer half-life - apparently as great as that of the proton - and there is some speculation that free protons might be more likely to decay over the eons than bound ones.
In GUT models, the exchange of an X or Y boson with the mass ΛGUT can lead to the last two operators suppressed by . The exchange of a triplet Higgs with mass M can lead to all of the operators suppressed by 1/M2. See doublet-triplet splitting problem.
In the absence of matter parity, supersymmetric extensions of the Standard Model can give rise to the last operator suppressed by the inverse square of sdown quark mass. This is due to the dimension-4 operators
The proton decay rate is only suppressed by which is far too fast unless the couplings are very small.