LHCb results and MicroBooNE results

[valenumr]

TL;DR Summary

LHCb results and MicroBooNE results

I'm curious to what folks thoughts are on the LHCb muon / electron decay ratio results are, as well as the MicroBooNE (null) results. Do the LHCb results really require a new force as the media seems to report? Do the MicroBooNE results indicate that a sterile heavy neutrino is dead?

 

[ohwilleke]

Two very distinct issues:

1. Lepton universality violations - I am not yet convinced that the anomalies reported are really due to New Physics beyond the Standard Model, but I'm trying to keep an open mind.

A lot of circumstances in which lepton universality violations could arise show no signs of these violations, despite the fact that all of the circumstances where they are purportedly seen are mediated in conventional Standard Model Theory by the same W and Z boson decays that show no violations elsewhere.

Why should this be observable in the form of an excess of electron events relative to muon events at statistically significant levels only in certain kinds of semi-leptonic B meson decays that involve what is usually assumed to involve only a two step weak force decays of a b quark?

For example, why doesn't it also show up in D meson decays? Why doesn't it show up in the semi-leptonic decays of baryons with one or more b quarks as valence quarks?

None of the theories advanced to explain these phenomena (e.g. leptoquarks) appears to me to be well motivated.

The statistical significance of these violations has been overstated by proponents of this theory who engaged in cherry picking without considering the look elsewhere effect properly, and by looking at benchmarks that they pretty much acknowledge are the wrong ones (e.g. looking at deviations of muon events and electron events in absolute terms from a one loop QCD prediction, rather than at the deviations of the ratio of event frequencies compared to the predicted value of the ratio of event frequencies).

I personally suspect that these are (1) statistical flukes (found only in the most rare kinds of hadron decays which since the raw numbers of law make it easier to statistical flukes to arise), (2) arise from some unconsidered process that the proponents think that they have adequately screened out with the event cuts but actually haven't properly excluded, or (3) arise from some other flaw in the calculation of the theoretical expectation to which the experimental results are being compared.

For example (for purposes of illustrating what a boring solution might look like, and not because I necessarily think that this particular solution is actually the correct one) suppose that the predicted number of electron events relative to muons events doesn't take into account a hadronic decay of a b quark, via a virtual top quark, into pions, that in turn decay to electrons (pions never or almost never decay into muons). Then, suppose further, that the electrons produced by the decay of these pions are not kept out of the data analyzed to look for lepton universality violations by the event selection cutoffs that have been used for some technical reason that the people on the team working on this problem who were supposed to consider this technical issue, forget to consider. Suddenly, you have a very prosaic explanation for why you have more electron events than you do muon events with no new physics. Mystery solved. Anomaly gone. No new physics required.

On the other hand, charged lepton universality violations aren't inherently such an absurd possibility. Neither the quarks, nor the uncharged leptons (i.e. neutrinos) display the universality of properties that charged leptons seem to show (i.e. having identical properties except for mass), and the evidence for some sort of violation hasn't vanished yet.

What is surprising, however, is that charged lepton universality violations would have such a large magnitude in semi-leptonic B meson decays, while not being detectible at statistically significant levels in any other context (many of which have been very precisely measured), without an obvious reason for the other contexts where lepton universality violations have not been observed to be different.

While I would not be surprised if there were very small charged lepton universality violations that could be seen only with ultra-precise measurements that crop up in many contexts, it is surprising to see a huge violation in one kind of interaction without seeing it anywhere else with anything like a comparable magnitude.

2. Sterile neutrinos and non-standard neutrino interactions - In my humble opinion, the evidence in support of the sterile neutrino hypothesis is too internally contradictory, too strongly contradicted and disfavored by other evidence, and too weakly theoretically grounded, to be correct. The evidence against this hypothesis is very strong, and there are plausible reasons that prior experiments could have seen what looked like a sterile neutrino observation but wasn't one.

Likewise, the evidence against "non-standard neutrino interactions" (NSI) is pretty overwhelming.