Proton decay

Proton decay is the key process to test the stability of matter and baryon number conservation and has long been a subject of both theoretical and experimental interests. Violation of baryon number conservation is one of the three key ingredients to explain the asymmetry of matter and antimatter in the Universe, as first formulated by Andrei Sakharov in 1967.

Despite significant experimental effort, proton decay has never been observed. The current experimental lower bound on the proton lifetime (${\displaystyle \tau _{p}}$) is 2.4×10 years (in the decay channel into a positron and a neutral pion: p → e + π).

According to the Standard Model, the proton, a type of baryon, is stable because baryon number is conserved. Since protons are the lightest baryons in the model, they cannot decay into other particles on their own and are therefore stable. However, baryon number conservation is an accidental global symmetry of the Standard Model, not associated with any fundamental gauge symmetry, slightly violated by non-perturbative SU(2)_Lsphaleron effects, negligible at low temperatures but relevant in the early Universe.

Positron emission and electron capture—forms of radioactive decay in which a proton becomes a neutron—are not proton decay, because in these processes the proton interacts with other particles within the atom.

Grand Unified Theories (GUTs) explicitly break the baryon number symmetry, allowing protons to decay via the Higgs particle, magnetic monopoles, or new X bosons with a half-life in the range 10 to 10 years. For comparison, the universe is roughly 1.4×10 (14 billion) years old, which is at least twenty orders of magnitude lower. Although at first glance it may seem nearly impossible to explore such long lifetimes, it is sufficient to monitor 10 protons per year (with efficiency = 1) to be sensitive to average lifetimes of 10 years. About 10 protons are contained in approximately 30 kton of water, slightly more than the fiducial volume of the most sensitive experiment currently in operation, Super-Kamiokande.

The easiest decay channel to detect is the one into a positron and a neutral pion, ${\displaystyle p ightarrow e^{+}\pi ^{0}}$, a common prediction of many GUT models. This channel has a very clean signature, with no invisible particles in the final state. This allows for full reconstruction of the proton mass from the decay products, and, since it is a two-body decay, the total recoil momentum is expected to be small.

Supersymmetric extensions of GUT models favor the ${\displaystyle p ightarrow {\overline { u }}_{\mu }K^{+}}$decay channel, which is more difficult to detect in water Cherenkov detectors because the antineutrino leaves the detector undisturbed and the kaon momentum is below the Cherenkov-light production threshold in water. The process can be identified by detecting the muons produced by kaon decays at rest, as well as the gamma rays emitted when a nucleon decays within the oxygen nucleus. The nucleus may remain in an excited state following this decay, from which it rapidly relaxes by emitting gamma rays.

Other processes can enable experiments to test the baryon number conservation, such as neutron-antineutron oscillations and specific nucleon–antinucleon conversion processes, accessible through electron–deuteron scattering.