Proton decay

Proton decay is a hypothetical transformation of a proton into lighter particles. It would violate baryon-number conservation and is predicted by some grand unified theories.

Baryon number: A quantum number that counts baryons minus antibaryons in a physical process. Proton decay requires interactions that do not conserve this quantity.
Super-Kamiokande: A large underground water Cherenkov detector in Japan used to study neutrinos and rare events. Its water target and photodetectors support leading searches for proton decay.
Proton: A positively charged composite particle made of two up quarks and one down quark. It is the particle whose hypothetical transformation defines proton decay.
Baryon asymmetry: The observed excess of matter over antimatter in the universe. Baryon-number violation may help explain this cosmic imbalance, though proton decay alone does not.
Grand unified theory: A particle-physics framework that seeks to unify the strong, weak, and electromagnetic interactions. Many such theories predict interactions capable of mediating proton decay.
Kamioka Nucleon Decay Experiment: An underground water Cherenkov experiment in Japan that searched for nucleon decay from 1983 to 1996. Its searches established early, influential limits on proton-decay rates.
Quark: An elementary particle that combines with other quarks to form hadrons. The proton's quark constituents must be rearranged or transformed in decay models.
Neutron-antineutron oscillation: A hypothetical transition in which a neutron becomes an antineutron. It violates baryon number like proton decay but does not require proton destruction.
X and Y bosons: Hypothetical massive gauge bosons in grand unified theories that can connect quarks and leptons. Their exchange can convert proton constituents into leptons.
Sudbury Neutrino Observatory: An underground heavy-water detector in Canada built to study solar neutrinos. Its heavy-water searches provided independent constraints on proton decay.

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