Monday, October 11, 2021

The Story Of A Random High Energy Physics Anomaly

One of the dirty little secrets of high energy physics is that there are an embarrassment of seemingly quite statistically significant discrepancies between theoretical expectations and experimental results (and not infrequently between different theoretical expectations). 

A typical paper describing such a discrepancy without unnecessary hype is this one from today's preprints.

It was made possible because the measurements of the decays at the Belle experiment was significantly more precise than the measurements made at the previous ALICE experiment which weren't sufficiently precise to cleanly distinguish experimental uncertainty from substantive differences between theoretical predictions and the experimental results at a sufficiently high statistical significance.

For the most part, the discrepancies usually arise because doing quantum chromodynamics (QCD) calculations and predictions (i.e. calculations involving the strong force of the Standard Model) is almost as much of an art as a science, since the only calculations that can be done involve approximations of the largely intractable exact equations of QCD, let alone the full Standard Model with all electroweak corrections as well. 

It is possible that some results may point to "new physics" beyond the Standard Model, but it is hard to know which discrepancies are real and which are flawed predictions. But, figuring out which oversimplifications do and don't matter is critical for distinguishing true anomalies from mere oversimplified methods for making theoretical calculations without undue computational effort.

It discusses the fraction of decays of some short lived spin-1/2 baryons (the analogs of protons and neutrons with heavier valence quarks) to particular decay products. There are more than six dozen possible decays of these charmed baryons (meaning that they have exactly one valence charm quark). 

This study focuses on two or three such possibilities in each case, with moderate frequency, in which a charm quark decays to an strange quark while emitting a W+ boson that decays to a charged anti-lepton and a neutrino of the same flavor as the charged anti-lepton. 

The positively charged charmed lambda baryons and neutral charmed chi baryons differ in that the former has an up quark in a position where the latter has a strange quark.

(1) positively charged charmed lambda baryons (valence quarks udc, a mass of about 2286 MeV, and a mean lifetime of about 202 femtoseconds) to a neutral lambda baryon (valence quarks uds, a mass of about 1116 MeV, and a mean lifetime of about 26,320 femtoseconds) together with a charged anti-lepton (i.e. a positron, anti-muon or anti-tau lepton) with the counterpart neutrino, and 

(2) the decay of a neutral charmed chi baryon (valence quarks dsc, a mass of about 2468 MeV with a mean lifetime of about 456 femtoseconds) to a negatively charged chi baryon (valence quarks dss, a mass of about 1322 MeV, and a mean lifetime of about 16,390 femtoseconds) together with together with a charged anti-lepton (i.e. a positron or anti-muon) with the counterpart neutrino. 

The strong parallels between these two kinds of decays suggests that their proportion of total decays should be similar to each other. But this naive observation wasn't as successful as expected.

The observed proportion of decays of these types is about a third of the theoretically predicted values in each case using a particularly simplified model (one based on SU(3) symmetry disregarding differences between the three lighter quark masses and using lambda decays as a benchmark), which is nominally well over five sigma in significance in two different related decays. But this is not very exceptional because this probably simply means that the theoretical calculation is off by a factor of three for some reason relating to its oversimplification.

The authors then speculate on what oversimplification used in their theoretical prediction was most likely to be the source of the problem with their theoretical prediction. 

After considering several possibilities, they suggest in their conclusion that the most likely issue was their initial neglect of the mass difference between the strange quark and the first generation up and down quark masses especially in the third valence quark position, while other oversimplifications of their theoretical prediction probably weren't nearly as important.

From the authors' perspectives, the paper's take away point is to recognize that there is a factor of three in the end result outcome in the predictions that flows from including this one additional complicating factor in the calculations. The implication is that this is a factor that one can't afford to ignore.

Another element of the analysis not focused upon by the authors is that both the theoretical result and the experimental data support the Standard Model model physics rules of "lepton universality" which means that electrons, muons and tau leptons differ from each other in only one property: mass, and that their antiparticle counterparts behave the same way, in a decay system with significant similarities to the small number of B meson decays in which lepton universality seems to be violated. In this case, there is no statistically significant difference between the rates of decays into different flavors of charged leptons.

Any theory explaining lepton universality violations has to explain why the vast majority of particle decays are consistent with lepton universality, while a few special cases seemingly are not. In other words, it has to figure out what makes the exceptions special.

The preprint and its abstract are as follows:

Xiao-Gang He, et al., "SU(3) symmetry and its breaking effects in semileptonic heavy baryon decays" arXiv:2110.04179 (October 8, 2021).

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