When Rutherford discovered the proton in 1919, the only other basic constituent of matter that was known of was the electron. There was no way that the proton could decay without violating charge conservation. Ten years later, Hermann Weyl went further, proposing the first version of what would become a law for baryon conservation. Even after the discoveries of the positron, and positive muons and pions – all lighter than the proton – there was little reason to question the proton’s stability. As Maurice Goldhaber famously pointed out, were the proton lifetime to be less than 1016 years we should feel it in our bones, because our bodies would be lethally radioactive. In 1954 he improved on this estimate. Arguing that the disappearance of a nucleon would leave a nucleus in an excited state that could lead to fission, he used the observed absence of spontaneous fission in 232Th to calculate a lifetime for bound nucleons of > 1020 years, which Georgy Flerov soon extended to > 3 × 1023 years.
Goldhaber also teamed up with Fred Reines and Clyde Cowan to test the possibility of directly observing proton decay using a 500 l tank of liquid scintillator surrounded by 90 photomultiplier tubes (PMTs) that was designed originally to detect reactor neutrinos. They found no signal, indicating that free protons must live for > 1021 years and bound nucleons for > 1022 years. By 1974, in a cosmic-ray experiment based on 20 tonnes of liquid scintillator, Reines and other colleagues had pushed the proton lifetime to > 1030 years.
Meanwhile, in 1966, Andrei Sakharov had set out conditions that could yield the observed particle–antiparticle asymmetry of the universe. One of these was that baryon conservation is only approximate and could have been violated during the expansion phase of the early universe. The interactions that could violate baryon conservation would allow the proton to decay, but Sakharov’s suggested proton lifetime of > 1050 years provided little encouragement for experimenters. This all changed around 1974, when proposals for grand unified theories (GUTs) came along. GUTs not only unified the strong, weak and electromagnetic forces, but also closely linked quarks and leptons, allowing for non-conservation of baryon number. In particular, the minimal SU(5) theory of Howard Georgi and Sheldon Glashow led to predicted lifetimes for the decay p → e+π0 in the region of 1031±1 years – not so far beyond the observed lower limit of around 1030 years.
This provided the justification for dedicated proton-decay experiments. By 1981 seven such experiments installed deep underground were using either totally active water Cherenkov detectors or sampling calorimeters to monitor large numbers of protons. These included the Irvine–Michigan–Brookhaven (IMB) detector based on 3300 tonnes of water and 2048 5-inch PMTs and KamiokaNDE in Japan with 1000 tonnes of water and 1000 20-inch PMTs. These experiments were able to push the lower limits on the proton lifetime to > 1032 years and so discount the viability of minimal SU(5) GUTs.
However, in 1987 IMB and Kamiokande II achieved greater fame by each detecting a handful of neutrinos from the supernova SN1987a. Kamiokande II was already studying solar and atmospheric neutrinos, but it was its successor, Super-Kamiokande, that went on to make pioneering observations of atmospheric and solar neutrino oscillations. And it is Super-Kamiokande that currently has the highest lower-limit for proton decay: 1.6 × 1034 years for the decay to e+π0.
Today, the theoretical development of GUTs continues, with predictions in some models of proton lifetimes up to around 1036 years. Future large neutrino experiments – such as DUNE, Hyper-Kamiokande and JUNO – feature proton decay among their goals, with the possibility of extending the limits on the proton lifetime to 1035 years. So the study of proton stability goes on, continuing the symbiosis with neutrino research.