Proton decay is a hypothetical transition of a proton into lighter particles that violates its ordinary baryon-number assignment. A possible searched-for channel is , followed by the neutral pion's decay into two photons. The existence and rate of any channel depend on interactions beyond the ordinary perturbative Standard Model processes.
A large water Cherenkov detector can search for the charged lepton and electromagnetic showers, reconstructing a candidate invariant mass and total momentum. Atmospheric-neutrino interactions can imitate parts of the signature, so background modeling and event selection are essential. Bound protons additionally involve nuclear motion and final-state interactions. Searches report channel-dependent limits using exposure and efficiency rather than assuming that every hypothetical baryon-violating operator gives the same lifetime.[ref]
Search for proton decay with an enlarged fiducial volume in Super-Kamiokande I-IV by Super-Kamiokande Collaboration. 2020 analysis: charged-lepton/neutral-pion modes, water-detector event selection and atmospheric-neutrino background
A proton-decay operator is a local effective interaction that violates baryon number. Standard Model gauge invariance first permits such operators at dimension six, including and .
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Proton decay is a hypothetical form of particle decay in which a proton, a fundamental constituent of atomic nuclei, transforms into lighter particles. This process would imply that protons are not absolutely stable, as is commonly assumed in the Standard Model of particle physics, which treats them as stable particles under normal conditions.
