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Anomalies persist in flavour-changing B decays

11 March 2020
The distribution of the angular variable P5’ as a function of the mass squared of the muon pair, q2. The LHCb Run 1 results (red), those from the additional 2016 dataset only (blue), and those from both datasets (black) are shown along with the SM predictions (orange). Credit: LHCb

The LHCb collaboration has confirmed previous hints of odd behaviour in the way B mesons decay into a K*and a pair of muons, bringing fresh intrigue to the pattern of flavour anomalies that has emerged during the past few years. At a seminar at CERN on 10 March, Eluned Smith of RWTH Aachen University presented an updated analysis of the angular distributions of B0→K*0μ+μ decays based on around twice as many events than were used for the collaboration’s previous measurement reported in 2015. The result reveals a mild increase in the overall tension with the Standard Model (SM) prediction, though, at 3.3σ, more data are needed to determine the source of the effect.

The B0→K*0μ+μ decay is a promising system with which to explore physics beyond the SM. A flavour-changing neutral-current process, it involves a quark transition (b→s) which is forbidden at the lowest perturbative order in the SM, and therefore occurs only around once for every million B decays. The decay proceeds instead via higher-order penguin and box processes, which are sensitive to the presence of new, heavy particles. Such particles would enter in competing processes and could significantly change the B0→K*0μ+μ decay rate and the angular distribution of its final-state particles. Measuring angular distributions as a function of the invariant mass squared (q2) of the muon pair is of particular interest because it is possible to construct variables that depend less on hadronic modelling uncertainties.

Potentially anomalous behaviour in an angular variable called P5′ came to light in 2013, when LHCb reported a 3.7σ local deviation with respect to the SM in one q2 bin, based on 1fb-1 of data. In 2015, a global fit of different angular distributions of the B0→K*0μ+μ decays using the total Run 1 data sample of 3 fb-1 reaffirmed the puzzle, showing discrepancies of 3.4σ (later reduced to 3.0σ when using new theory calculations with an updated description of potentially large hadronic effects). In 2016, the Belle experiment at KEK in Japan performed its own angular analysis of B0→K*0μ+μ using data from electron—positron collisions and found a 2.1σ deviation in the same direction and in the same q2 region as the LHCb anomaly.

We as a community have been eagerly waiting for this measurement and LHCb has not disappointed

Jure Zupan

The latest LHCb result includes additional Run 2 data collected during 2016, corresponding to a total integrated luminosity of 4.7fb-1. It shows that the local tension of P5′ in two q2 bins between 4 and 8 GeV2/c4 reduces from 2.8 and 3.0σ, as observed in the previous analysis, to 2.5 and 2.9σ. However, a global fit to several angular observables shows that the overall tension with the SM increases from 3.0 to 3.3σ. The results of the fit also find a better overall agreement with predictions of new-physics models that contain additional vector or axial-vector contributions. However, the collaboration also makes it clear that the discrepancy could be explained by an unexpectedly large hadronic effect that is not accounted for in the SM predictions.

“We as a community have been eagerly waiting for this measurement and LHCb has not disappointed,” says theorist Jure Zupan of the University of Cincinnati. “The new measurements have moved closer to the SM predictions in the angular observables so that the combined significance of the excess remained essentially the same. It is thus becoming even more important to understand well and scrutinise the SM predictions and the claimed theory errors.”

Flavour puzzle
The latest result makes LHCb’s continued measurements of lepton-flavour universality even more important, he says. In recent years, LHCb has also found that the ratio of the rates of muonic and electronic B decays departs from the SM prediction, suggesting a violation of the key SM principle of lepton-flavour universality. Though not individually statistically significant, the measurements are theoretically very clean, and the most striking departure – in the variable known as RK — concerns B decays that proceed via the same b→s transition as B0→K*0μ+μ. This has led physicists to speculate that the two effects could be caused by the same new physics, with models involving leptoquarks or new gauge bosons in principle able to accommodate both sets of anomalies.

An update on RK based on additional Run 2 data is hotly anticipated, and the collaboration is also planning to add data from 2017-18 to the B0→K*0μ+μ angular analysis, as well as working on further analyses with b-quark transitions in mesons. LHCb also recently brought the decays of beauty baryons, which also depend on b→s transitions, to bear on the subject. Departures from the norm have also been spotted in B decays to D mesons, which involve tree-level b→c quark transitions. Such decays probe lepton-flavour universality via comparisons between tau leptons and muons and electrons but, as with RK, the individual measurements are not highly significant.

“We have not seen evidence of new physics, but neither were the B physics anomalies ruled out,” says Zupan of the LHCb result. “The wait for the clear evidence of new physics continues.”

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