New data strengthens R_K flavour anomaly

The principle that the charged leptons have identical electroweak interaction strengths is a distinctive feature of the Standard Model (SM). However, this lepton-flavour universality (LFU) is an accidental symmetry in the SM, which may not hold in theories beyond the SM. The LHCb collaboration has used a number of rare decays mediated by flavour-changing neutral currents, where the SM contribution is suppressed, to test for deviations from LFU. During the past few years, these and other measurements, together with results from B-factories, hint at possible departures from the SM.

In a new measurement of a LFU-sensitive parameter “R_K” with increased precision and statistical power, reported today at the Rencontres de Moriond, LHCb has strengthened the significance of the flavour anomalies. The value R_K  probes the ratio of B-meson decays to muons and electrons: R_K = BR(B⁺→K⁺μ⁺μ^–)/BR(B⁺→K⁺e⁺e^–). Testing LFU in such b→sℓ⁺ℓ^– transitions has the advantage that not only are SM contributions suppressed, but the theoretical predictions are very precise. Therefore, any significant deviation of R_K from unity would imply physics beyond the SM.

The experimental challenge lies in the fact that, while electrons and muons interact via the electroweak force in the same way, the small electron mass means it interacts with detector material much more than muons. For example, electrons radiate a significant number of bremsstrahlung photons when traversing the LHCb detector, which degrades reconstruction efficiency and signal resolution compared to muons. The key to control this effect is to use the decays J/ψ→e⁺e^– and J/ψ→μ⁺μ^–, which are known to have the same decay probability and can be used to calibrate and test electron reconstruction efficiencies. High precision tests with the J/ψ are compatible with LFU, which provides a powerful cross-check on the experimental analysis.

Previous LHCb measurements of R_K and R_K* (which probes B⁰→K*ℓ⁺ℓ^– decays) in 2019 and 2017 respectively, provide hints of deviations from unity. The latest analysis of R_K, which uses the full dataset collected by the experiment in Run 1 and Run 2 of the LHC, represents a substantial improvement in precision on the previous measurement (see figure) thanks to doubling the dataset. The R_K ratio is measured to be three standard deviations from the SM prediction (see figure). This is the first time that a departure from LFU above this level has been seen in any individual B-meson decay, with a value of R_K=0.846^+0.042_-0.039 (stat.) ^+0.013_-0.012 (syst.).

Although it is too early to conclude anything definitive at this stage, this deviation is consistent with a pattern of anomalies which have manifested themselves in b→s ℓ⁺ℓ^– and similar processes over the course of the past decade. In particular, the strengthening R_K anomaly may be considered alongside hints from other measurements of these transitions, including angular asymmetries and decay rates.

The LHCb experiment is well placed to clarify the potential existence of new-physics effects in these decays. Updates on a suite of  b→s ℓ⁺ℓ^– related measurements with the full Run 1 and Run 2 dataset are underway. A major upgrade to the detector during the ongoing second long shutdown of the LHC will offer a step change in precision in Run 3 and beyond.