Monday, June 25, 2018

B Quark Decays Still Anomalous

For the most part, the Standard Model is well behaved. Indeed, it is so well behaved that they call it the nightmare scenario for physicists who were suspecting some surprises at the LHC. But, there are still some anomalies out there.

The magnetic moment of the muon (muon g-2) still isn't quite right, and as noted in previous posts this month, new experiments may see if that was just an experimental measurement error.

The size of a hydrogen atom with muons instead of electrons in its shell isn't quite what we have predicted it to be, and scientists are looking into that.

And then, there are b quark decays, which don't seem to be behaving quite as we'd expect them to, seemingly violating "lepton universality", which is strictly observed to high precision in other experiments.
A test case for the bottom-up methodology is the bottom meson, a composite particle made of something called a bottom quark and another known as a lighter quark. Bottom mesons appear to be decaying with the ‘wrong’ probabilities. Experiments in the LHC have measured billions of such decays, and it seems that the probability of getting a muon pair from particular interactions is about three-quarters of the probability of what the Standard Model says it should be. We can’t be totally sure yet that this effect is in strong disagreement with the Standard Model – more data is being analysed to make sure that the result is not due to statistics, or some subtle systematic error.
Some of these anomalies will turn out to be statistical flukes or subtle systemic errors. But, physicists can always hope. The resolutions of these anomalies, however, if they do come from beyond the Standard Model physics, are not obviously resolutions that come from the "usual suspects" of popular beyond the Standard Model physics hypotheses. 

It is also curious that all three of these leading anomalies in particle physics involve muons. I'm not sure what to make of that, but it is worth putting out there. It could be as simple as the fact that the properties of the muon can be predicted with extraordinary precision in the Standard Model, so that even slight discrepancies that could have all sorts of sources that can usually be ignored, could be at fault. The only way we can know for sure is to keep doing science.

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