AWhat changed
The Super-Kamiokande Collaboration searched its 1996–2018 data for proton decays into a positron or antimuon and a neutral pion. Using 450 kiloton-years of exposure, the collaboration found no significant excess over atmospheric-neutrino backgrounds and reported 90% confidence lower limits on the partial lifetimes of 2.4 × 10³⁴ and 1.6 × 10³⁴ years for the respective channels. A later search of overlapping pure-water data in 2026 targeted different three-body channels containing two neutral pions and likewise found no significant excess; its reported 90% lower limits were 7.2 × 10³³ and 4.5 × 10³³ years for the positron and antimuon channels.
Scientific scope
The Super-Kamiokande Collaboration searched its 1996–2018 data for proton decays into a positron or antimuon and a neutral pion. Using 450 kiloton-years of exposure, the collaboration found no significant excess over atmospheric-neutrino backgrounds and reported 90% confidence lower limits on the partial lifetimes of 2.4 × 10³⁴ and 1.6 × 10³⁴ years for the respective channels. A later search of overlapping pure-water data in 2026 targeted different three-body channels containing two neutral pions and likewise found no significant excess; its reported 90% lower limits were 7.2 × 10³³ and 4.5 × 10³³ years for the positron and antimuon channels. These are lower limits on lifetime divided by the branching ratio for two specific decay modes, not an observation of proton decay or proof that all proton decay is impossible. The positron channel had zero candidate events; the antimuon channel had one candidate consistent with the estimated background, the same event as in the preceding related search. The larger 2020 analysis reuses earlier data, so earlier limits are not counted as independent discoveries. Its Bayesian calculation assumes a uniform decay-rate prior and relies on modeled signal efficiency, backgrounds and systematic uncertainties. Raw event reconstruction and model code were not independently replayed. The 2026 three-body analysis uses a conventional 22.5 kiloton fiducial region and 0.401 megaton-years, not the larger 2020 exposure; it found one candidate in each channel, both consistent with the corresponding expected backgrounds. The analysis uses a uniform prior on decay rate. The cited v2 manuscript and the later journal version may differ. Table I lists SK-VI as 2020–2020 despite 564.4 live days; SK-VI is outside the analyzed I–V data and the inconsistent printed date is not adopted.
Evidence
The linked original works support the scoped statement and contribution attribution. WRS has not independently replayed experiments or reconstructed a full mathematical proof. These are lower limits on lifetime divided by the branching ratio for two specific decay modes, not an observation of proton decay or proof that all proton decay is impossible. The positron channel had zero candidate events; the antimuon channel had one candidate consistent with the estimated background, the same event as in the preceding related search. The larger 2020 analysis reuses earlier data, so earlier limits are not counted as independent discoveries. Its Bayesian calculation assumes a uniform decay-rate prior and relies on modeled signal efficiency, backgrounds and systematic uncertainties. Raw event reconstruction and model code were not independently replayed. The 2026 three-body analysis uses a conventional 22.5 kiloton fiducial region and 0.401 megaton-years, not the larger 2020 exposure; it found one candidate in each channel, both consistent with the corresponding expected backgrounds. The analysis uses a uniform prior on decay rate. The cited v2 manuscript and the later journal version may differ. Table I lists SK-VI as 2020–2020 despite 564.4 live days; SK-VI is outside the analyzed I–V data and the inconsistent printed date is not adopted.
Contributors
- The named authors in the research record · SCIENCE: Credit belongs to the cited collaborations. The individual author list is incomplete; no sole discoverer or individual effort share is inferred.
- Search for proton decay via p→e⁺π⁰ and p→μ⁺π⁰ with an enlarged fiducial volume in Super-Kamiokande I–IV
- Search for proton decay via p→e⁺π⁰π⁰ and p→μ⁺π⁰π⁰ in 0.401 megaton-years exposure of Super-Kamiokande I–V
Provisional WRS assessment, edition 2026-10-28.1. Independent expert calibration is pending. This grade does not certify correctness or assign ownership.
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