Complementing previous results by Belle, BaBar and LHCb, the LHCb collaboration has reported a new test of lepton flavour universality in b → cℓ νℓ decays. At a seminar at CERN on Tuesday 18 October, the collaboration announced the first simultaneous measurements of the ratio of the branching fraction of B-meson decays to D mesons: R(D*)= BR(B→D*τ–ντ)/BR(B→D*μ–νμ) and R(D)= BR(B–→D0τ–ντ)/BR(B–→D0μ–νμ) at a hadron collider. Based on Run 1 data recorded at a centre-of-mass energy of 7 and 8 TeV, they found R(D*) = 0.281 ± 0.018 (stat.) ± 0.024 (syst.) and R(D) = 0.441 ± 0.060 (stat.) ±0.066 (syst.). The values, which are consistent with the Standard Model (SM) expectation within 1.9 σ, bring further information to the pattern of “flavour anomalies” reported in recent years.
Lepton-flavour universality holds that aside from mass differences, all interactions must couple identically to different leptons. As such, the rate of B-meson decays to different leptons is expected to be the same, apart from known differences due to their different masses. Global fits of R(D(*)) measurements, which probe b → c quark transitions, show that the ratio of B-meson to D-meson decays tends to be larger (by about 3.2 σ) than the SM prediction. The ratios of electronic to muonic B-meson decays, R(K), which probe b → s quark transitions, are also under scrutiny to test this basic principle of the SM.
To reconstruct b → cτ– ντ decays, LHCb used the leptonic τ–→μ–νν decay to identify the visible decay products D(*) and µ–. “We use the measurement of the B flight direction to constrain the kinematics of the unreconstructed particles, and with an approximation reconstruct the rest frame kinematic quantities,” says LHCb’s Greg Ciezarek, who presented the results. “The challenge is then to understand the modelling of the various background processes which also produce the same visible decay products but have additional missing particles different distributions in the rest frame quantities. We use control samples selected based on these missing particles to constrain the modelling of background processes and justify our level of understanding.”
The respective SM predictions for the ratios R(D) and R(D*) are very clean because they are independent of uncertainties induced by the CKM-matrix element Vcb and hadronic matrix elements. The new values of R(D) and R(D*) are compatible both with the current world average compiled by the HFLAV collaboration, and with the SM prediction (at 2.2σ and 2.3σ). The combined LHCb result provides improved sensitivity to a possible lepton-universality breaking process.
“Rare B-meson decays and ratios such as R(K) and R(D(*)) are powerful probes to search for beyond the Standard Model particles, which are not directly detectable at the LHC,” says Ben Allanach, theorist at the University of Cambridge.
