The problem
Protons make up part of ordinary matter. Could they eventually decay through events so rare that none has been established? One search looks for a proton turning into a positron, the electron’s positively charged counterpart, and a neutral pion, a short-lived particle that produces two high-energy photons. In a large water detector, the resulting particle showers can leave recognizable light patterns. Finding a pattern is only a candidate; ordinary particle interactions can imitate it.[1] [2]
A proton in water could, under some extensions of particle physics, decay into a positron and a neutral pion; the pion then produces two gamma-ray photons. Do any such proton-decay channels exist, and what are their rates?
In a water Cherenkov detector, sufficiently fast charged particles emit light that appears as rings on surrounding sensors. The positron and the photons’ secondary charged-particle showers produce shower-like rings; the photons do not directly emit Cherenkov light. The Super-Kamiokande I–IV search selects two- or three-ring events, reconstructed masses and low total momentum consistent with the candidate final state. Nuclear interactions, ring overlap and detector efficiency affect the selection, and atmospheric-neutrino events can imitate it.
The 2020 manuscript’s positron-plus-pion search found no candidates in data from 1996–2018. It sets a lower bound on partial lifetime, meaning total lifetime divided by the fraction of decays into this channel. The statistical inference uses exposure, signal efficiency, expected backgrounds and their uncertainties. Watching a huge number of protons for a limited time can constrain a rare event rate without waiting for one individual proton’s full lifetime; it cannot prove that all decay channels are absent. These decays would violate baryon number, the accounting in which a proton counts as one baryon and the stated outgoing particles count as none.[1] [2]
Scope
Experimental tests of proton stability and baryon-number-violating decay modes predicted by some extensions of particle physics.
What remains open
Why it matters
Definitions
3 terms
Cherenkov light
Light emitted by a charged particle moving faster than light propagates in the detector medium. Its ring pattern helps reconstruct an event; high-energy photons are detected through secondary charged-particle showers.[2]
Partial lifetime
For a specified decay channel, the total lifetime divided by the branching fraction—the fraction of all decays that use that channel. A finite lower bound constrains its rate without proving absolute stability.[2]
Sources
- Physics | Hyper-Kamiokande
Notes on Physics | Hyper-Kamiokande
The collaboration explains proton-stability searches, an example positron/pion channel and their connection to unification proposals. Future sensitivities are program goals, not observations.
- Search for Proton Decay via p→e⁺π⁰ and p→μ⁺π⁰ with an Enlarged Fiducial Volume in Super-Kamiokande I–IV
Notes on Search for Proton Decay via p→e⁺π⁰ and p→μ⁺π⁰ with an Enlarged Fiducial Volume in Super-Kamiokande I–IV
The exact 2020 v1 manuscript provides the positron/pion search signature, Cherenkov reconstruction, background/efficiency treatment and finite channel-specific partial-lifetime inference.