Still No Sign of Proton Decay

The Super-Kamiokande Experiment has used an independent method to exclude proton decay and related di-nucleon decays involving protons to charged leptons, neutrinos, photons and neutral and invisible to the experiment's detector's X particles, up to very large mean lifetimes on the order of 10³¹ to 10³² years. By comparison, the universe is about 1.3*10¹⁰ years old.

So, over the life of the universe, the fraction of protons that decay as measured by these methods is less than one per 10²¹ protons (in words, less than one hundred per gram of protons, and probably less than ten per gram of protons).

Previous studies using other methods have set a minimum proton lifetime on the order of 10³³ to 10³⁴ years, i.e. 1.0 to 0.1 such decays per gram of protons over the entire lifetime of the universe, or put another way, less than 1 such decay per 10 kilotons of protons per year (a measurement of truly stunningly great precision).  A kiloton of protons is pretty much indistinguishable in mass from a kiloton of hydrogen at this level of precision in measurement.

All of the decays searched for in this study would violate baryon number conservation (which is perfectly conserved in the Standard Model) and also B-L number conservation (a widely assumed alternative), unless the missing X particles had a baryon number of 2 (in which case lepton number is also violated by a factor of -1).  Violations of these conserved quantity could explain the matter-antimatter asymmetry of the universe in the right amounts and the right types, assuming that the baryon number and lepton number of the universe were zero at t=0 in the Big Bang scenario, something widely assumed on aesthetic grounds by theoretical physicists.

But, the di-nucleon decays would not violate lepton number conservation, and the proton decays would violate lepton number conservation only if the X particles in the interaction (e.g. a heavy sterile neutrino), which were indirectly detected using missing energy, did not have a positive lepton number to counterbalance the charged anti-leptons to which they would be hypothetically paired.

The decays searched for would also not violate conservation of electric charge.

The X particles searched for would also violate "R-parity" or the equivalent, which is a conservative or approximately conserved quantity in many supersymmetry (SUSY) models.

The result is consistent with the Standard Model physics expectation, which is zero.  But this experiment, while not improving the overall limit on proton decay very much, makes the findings from other experiments measuring the same thing in different ways more robust.

Many Grand Unified Theories a.k.a GUTs, that try to unify the three Standard Model forces, including the simplest of them that can include all three Standard Model forces and all of the Standard Model particles, which is an SU(5) GUT model, predict proton decay in some form at detectable levels, and can be experimentally excluded on that basis.  Many remaining GUT theories predict proton decay at rates just slightly slower than the current reach of experimental evidence.

The stunningly low experimentally measured rate (relative to the stability of any other kinds of composite particles in the universe) clearly establishes that proton decay has had inconsequential phenomenological impact on the history of the universe for the vast majority of the 13.8 billion year history of the universe.

But, if proton decay happened more readily at a threshold energy scale in the extremely early stage of the universe, it might produce some sort of thermal relic dark matter in large enough quantities to impact cosmology.  The tight bounds on proton decay, however, greatly limit any realistic versions of these scenarios.

Other Physics News

Meanwhile, the ATLAS experiment has once again ruled out "flavor changing neutral currents" also forbidden in the Standard Model, to an every greater level of precision, with branching ratios limited to less than one per one thousand decays in an interaction where FCNC's have been proposed to exist on a highly suppressed basis in some Beyond the Standard Model theories.

These core qualitative assumptions of the Standard Model have remained very robust.