= Proton decay
{wiki}
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 $p\to e^++\pi^0$, 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.<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]{ref}
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