and decays into a positron and a neutral pion that itself immediately decays into two gamma ray photons: Since a positron is an antilepton this decay preserves number, which is conserved in most GUTs. Additional decay modes are available (e.g.: ), 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 (GUTs) such as the Georgi–Glashow model, which were the first consistent theories to suggest proton decay, postulated that the proton's half-life would be at least years. As further experiments and calculations were performed in the 1990s, it became clear that the proton half-life could not lie below years. Many books from that period refer to this figure for the possible decay time for baryonic matter. More recent findings have pushed the minimum proton half-life to at least ~ years, ruling out the simpler GUTs (including minimal SU(5) / Georgi–Glashow) and most non-SUSY models. The maximum upper limit on proton lifetime (if unstable), is calculated at a bound applicable to SUSY models, with a maximum for (minimal) non-SUSY GUTs at 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 antineutrino) in a process called beta decay. Free neutrons have a half-life of 10 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. Projected proton lifetimes The lifetime of the proton in vanilla SU(5) can be naively estimated as . Supersymmetric GUTs with reunification scales around yield a lifetime of around 1034 yr, roughly the current experimental lower bound. Decay operators Dimension-6 proton decay operators The dimension-6 proton decay operators are , , and where is the cutoff scale for the Standard Model. All of these operators violate both baryon number (B) and lepton number (L) conservation but not the combination B − L. 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