 Hypothetical particle decay process of a proton |
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 1967.1
This article is about the hypothetical decay of protons. For the type of radioactive decay in which a nucleus ejects a proton, see Proton emission. For the radioactive decay where a proton within a nucleus converts to a neutron, see positron emission.
Despite significant experimental effort, proton decay has never been observed. The current experimental lower bound on the proton lifetime () is 2.4×1034 years (in the decay channel into a positron and a neutral pion: p → e+ + π0).2
According to the Standard Model, the proton, a type of , is stable because is . 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 effects, negligible at low temperatures but relevant in the early Universe.