LHCb digs deeper into lepton-flavour universality

13 October 2017

The LHCb collaboration has released yet another result in its campaign to test lepton-flavour universality. Following anomalies already detected in the rate that B mesons decay into muons compared to electrons or tau leptons, the latest result concerns the charmed B meson, B+c.


Using data recorded at collision energies of 7 and 8 TeV during LHC Run 1, LHCb reports evidence for the semi-tauonic decay B+c J/ψτ+ντ and has performed a measurement of the ratio R(J/ψ) = Br(B+c J/ψτ+ντ)/Br(B+c J/ψμ+ νμ). The ratio is found to be 0.71±0.17±0.18, which is within 2σ of the expected Standard Model (SM) range of 0.25–0.28. The SM prediction of R(J/ψ) deviates from unity only due to the large mass difference between the tau and the muon.

Both the semi-muonic decay, B+c J/ψμ+ νμ, and the semi-tauonic decay, B+c J/ψτ+ντ, with J/ψ→ μ+μ and τ+ → μ+νμ ντ, lead to a three-muon final state with the two channels distinguished by their decay kinematics. Despite the distinct signature, however, the analysis must overcome several major challenges. For example, since Bc mesons account for less than 0.1% of the b hadrons produced at the LHC energies, light b hadrons are a major source of background when one or more particles in their final states are misidentified in the detector. Fortunately, the presence of two heavy quarks in the Bc means it decays nearly three times faster than its lighter cousins, providing a powerful handle for statistically separating their respective contributions.

The latest LHCb result adds to the intriguing picture emerging from the measurements of semi-tauonic decays of b-flavoured hadrons. Previous studies of the ratios of branching fractions between B → D(*) τ+ ντ and B → D(*) μ+ νμ at LHCb, BaBar and Belle have shown hints of departure from lepton-flavour universality. The combined effect is now almost at the level of 4σ with respect to the SM prediction. In addition, previous LHCb analyses of B → K(*) μ+μ and B → K(*) e+e decays also deviate from the SM by about 2.5σ.

There is much more to come from LHCb on tests of lepton-flavour universality, which remains one of the most enduring hints of deviation from the SM. This includes updates of the results with Run-2 data and measurements from other b-hadron species.

• This article was corrected on 10 November 2017.

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