Thursday, January 18, 2018

Maybe b quark decays aren't weird after all

Some indications that lepton universality (i.e. the propositions that leptons are identical to each other in their weak force decay behavior except that they are different in mass), is violated in b quark decays, which shouldn't happen in the Standard Model of Particle Physics, has created the latest cottage industry of "new physics" models that could explain this phenomena. But, it turns out that most of these models are tightly constrained or ruled out entirely by other data. 

Also, more accurate calculations of the Standard Model prediction in the "non-perturbative" regime governed by lattice QCD make the observed anomalies appear smaller than they had seemed using only perturbative QCD Standard Model predictions near the edge of their domain of applicability. 

One of the latest papers reviewing these constraints, from a month ago, has the following abstract:

"Many new physics models that explain the intriguing anomalies in the b-quark flavour sector are severely constrained by Bs-mixing, for which the Standard Model prediction and experiment agreed well until recently. New non-perturbative calculations point, however, in the direction of a small discrepancy in this observable. Using up-to-date inputs to determine ΔMSMs, we find a severe reduction of the allowed parameter space of Z and leptoquark models explaining the B-anomalies. Remarkably, in the former case the upper bound on the Z mass approaches dangerously close to the energy scales already probed by the LHC. We finally identify some model building directions in order to alleviate the tension with Bs-mixing."

Luca Di Luzio, Matthew Kirk and Alexander Lenz, "One constraint to kill them all?" (December 18, 2017).

Another paper from two weeks ago also constrains potential remedies for these anomalies by another means:

B decays proceeding via bcν transitions with =e or μ are tree-level processes in the Standard Model. They are used to measure the CKM element Vcb, as such forming an important ingredient in the determination of e.g. the unitarity triangle; hence the question to which extend they can be affected by new physics contributions is important, specifically given the long-standing tension between Vcb determinations from inclusive and exclusive decays and the significant hints for lepton flavour universality violation in bcτν and bs decays. We perform a comprehensive model-independent analysis of new physics in bcν, considering vector, scalar, and tensor interactions, including for the first time differential distributions of BDν angular observables. We show that these are valuable in constraining non-standard interactions. Specifically, the zero-recoil endpoint of the BDν spectrum is extremely sensitive to scalar currents, while the maximum-recoil endpoint of the BDν spectrum with transversely polarized D is extremely sensitive to tensor currents. We also quantify the room for e-μ universality violation in bcν transitions, predicted by some models suggested to solve the bcτν anomalies. Specific new physics models, corresponding to all possible tree-level mediators, are also discussed. As a side effect, we present Vcb determinations from exclusive B decays, both with frequentist and Bayesian statistics, leading to compatible results. The entire numerical analysis is based on open source code, allowing it to be easily adapted once new data or new form factors become available.

Martin Jung, David M. Straub, "Constraining new physics in b→cℓν transitions" (January 3, 2018).

In short, the evidence continues to mount that the observed b decay anomalies are a fluke or arise either from experimental errors or inaccuracies in theoretical Standard Model predictions, rather than from "new physics."