Are diamonds for ever, or do protons decay? A tale of the unexpected
Prof. Donald H. Perkins, St. Catherine's College, Oxford
Our picture of the universe is based on the concept that matter is everlasting. In deed, our very existence shows that, if the protons of matter do decay, their lifetime must be over 106 times the age of the universe. We postulate a conserved bary on number, so baryon and antibaryon can be created as a pair, but never singly. This has to be contrasted with the pronounced asymmetry we find in the universe at large, where baryons outnumber antibaryons by at least a factor of 10,000. This asymmetry is an outstanding and so far unsolved puzzle.
The discovery of neutral weak currents in 1973 validated the electroweak model, and a natural extension was that to a grand unified theory (GUT) in which the strong and electroweak fields merge into a unified interaction at about 1014 GeV. Leptons and quarks can then transform one into another. The result is that protons are then predicted to decay at a known rate.
This early GUT model received strong support from results of neutrino experiments in the late 1970s - which turned out later to be wrong! - and led to a proton lifetime prediction of about 1030 years. This meant that, in a thousand tons of material, one proton should decay every day, and led to kiloton-mass, deep underground experiments. But no proton decays have been found so far and the observed limit on the lifetime now exceeds 1033 years.
But all was not lost. Coming down to Earth from interactions of primary cosmic rays inthe atmosphere were neutrinos producing ‘background’ interactions which in fact gave some of the first evidence for neutrino mass and of new physics beyond the Standard Model. The failed search for proton decay has led to these new and unexpected developments. In a full circle, these tiny neutrino masses can be attributed to mixing with very massive neutrinos on the GUT mass scale, and decays of such massive neutrinos may be responsible for the matter asymmetry of the universe.