LHCb digs deeper into lepton-flavour universality

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/ψ τ⁺ ν_τ and has performed a measurement of the ratio R(J/ψ) = Br(B⁺_c→J/ψ τ⁺ ν_τ)/Br(B⁺_c→J/ψ μ⁺ν_μ). 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/ψ μ⁺ν_μ, and the semi-tauonic decay, B⁺_c→J/ψ τ⁺ ν_τ, with J/ψ → μ⁺μ^– and τ⁺→ μ⁺ ν_μν_τ, 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 B_c 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 B_c 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⁺ν_τ and B → D^(*)μ⁺ν_μ 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.