Many grand unified theories of physics propose that protons, while long lived, decay eventually. So far, experimental measurements find no evidence of this happening, as the Standard Model of Particle Physics predicts (by virtue of the requirement of conservation of baryon number).
In particular, this rules out many SU(5) GUT theories, including the simplest ones.
We searched for proton decay viap→μ+K0 in 0.37 Mton⋅ years of data collected between 1996 and 2018 from the Super-Kamiokande water Cherenkov experiment. The selection criteria were defined separately forK0S andK0L channels. No significant event excess has been observed. As a result of this analysis, which extends the previous search by an additional 0.2 Mton⋅ years of exposure and uses an improved event reconstruction, we set a lower limit of3.6×1033 years on the proton lifetime.
Super-Kamiokande Collaboration, "Search for proton decay via p→μ+K0 in 0.37 megaton-years exposure of Super-Kamiokande" arXiv:2208.13188 (August 28, 2022).
The universe is about 1.37 x 10^10 years old. This limitation means that we would expect that not more than about 2 in 10^23 protons would have decayed so far, everywhere in the universe, for all time. And, of course, this is just a lower bound on the proton lifetime.
The hypothesis that the proton is completely stable seems far more plausible.
2 comments:
though just to mention, even in standard model there is baryon number violation (though its in units of 3, 3 protons -> 1 spharelon) though its super suppressed
I don't disagree and have blogged on sphaleron interactions before, although it isn't rightly called proton decay.
In theory, those interactions are possible at energies about 100x those of the LHC, and I've done back of napkin calculations to confirm that the energy densities involved they wouldn't necessarily give rise to black holes.
Still, given that the energy-density involved is much greater than, for example, a neutron star, it is possible that this theoretical possibility can't be realized due to another constraint not yet discovered, such as a maximum energy-density limit.
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