Tau.Neutrino said:
Physicists review three experiments that hint at a phenomenon beyond the Standard Model of particle physics
To anyone but a physicist, it sounds like something out of “Star Trek.” But lepton universality is a real thing.
It has to do with the Standard Model of particle physics, which describes and predicts the behavior of all known particles and forces, except gravity. Among them are charged leptons: electrons, muons and taus.
A fundamental assumption of the Standard Model is that the interactions of these elementary particles are the same despite their different masses and lifetimes. That’s lepton universality. Precision tests comparing processes involving electrons and muons have not revealed any definite violation of this assumption, but recent studies of the higher-mass tau lepton have produced observations that challenge the theory.
Read more at: https://phys.org/news/2017-06-physicists-hint-phenomenon-standard-particle.html#jCp
Measurements of muons also “hint” at physics beyond the standard model, in at least two different ways. But like the tau, not a high enough statistical significance, it could just be random variation. One of the two ways is “muon anomalous magnetic moment”. Wikipedia says of this: “As of November 2006, the measurement disagrees with the Standard Model by 3.4 standard deviations, suggesting physics beyond the Standard Model may be having an effect (or that the theoretical/experimental errors are not completely under control). This is one of the long-standing discrepancies between the Standard Model and experiment.” A more accurate measurement is starting this year.
> The rates for some decays involving the heavy lepton tau, relative to those involving the light leptons—electrons or muons—were higher than the Standard Model predictions. … the three experiments, taken together, demonstrate a stronger result that challenges lepton universality at the level of four standard deviations, which indicates a 99.95 percent certainty.
For particle physics, four standard deviations is good. But what does “lepton universality” mean?
It’s all very well to point out a deviation from the standard model, but it has to be a deviation from the standard model that makes sense in some context or other. There are hundreds of theories that extend the standard model in some way or another, does this agree with any of them?