LHCb brings leptons into line

At a seminar held at CERN today, the LHCb collaboration presented new measurements of rare B-meson decays that provide a high-precision test of lepton flavour universality, a key feature of the Standard Model (SM). Previous studies of these decays had hinted at intriguing tensions with predictions, but the results of an improved and wider-reaching analysis of the full LHCb dataset are in agreement with the SM.

A central mystery of particle physics is why the 12 elementary quarks and leptons are arranged in pairs across three generations, identical in all but mass. Lepton flavour universality (LFU) states that the SM gauge bosons are indifferent to which generation a charged lepton belongs, implying that certain decays of hadrons involving leptons from different generations should occur at the same rates. In recent years, however, an accumulation of results has suggested a possible violation of LFU in B-meson decays involving fundamental b- to s-quark transitions, such as the decay of a B into a K meson. Such processes are highly suppressed in the SM because they proceed through higher-order diagrams, making them promising channels in which to detect the possible influence of new particles.

A powerful test of LFU is to measure the relative rates of the processes B → Kμ⁺μ^– and B → Ke⁺e^–, a quantity called R(K), and the equivalent ratio for decays involving an excited kaon, R(K*). The SM predicts such ratios to be equal to unity once differences in the lepton masses are accounted for. In 2021, based on data collected during LHC Run 1 and Run 2, LHCb found R(K) to lie 3.1 σ below the SM prediction. For R(K*), measurements in 2017 based on Run 1 data were consistent with the SM at the level of 2–2.5 σ.

Earlier LHCb indications of anomalies with lepton flavour universality triggered immense excitement

The latest LHCb analysis simultaneously measures R(K) and R(K*) using the full Run 1 and Run 2 datasets. A sequence of multivariate selections and strict particle-identification requirements produced a higher signal purity and a better statistical sensitivity than the previous analysis. The two ratios were also computed in two bins of the squared di-lepton momentum-transfer q², thereby producing four independent measurements. The measured values of R(K) and R(K*) are now compatible with the SM within 1 σ and supersede previous LHCb publications on these topics. The new value of R(K*) is based on an integrated luminosity three times larger than that used in 2017, and the two results are in broad agreement. For R(K) in the central q² region, on the other hand, the new value is significantly higher than the 2021 result.

“Although a component of this shift can be attributed to statistical effects, it is understood that this change is primarily due to systematic effects,” explains LHCb spokesperson Chris Parkes of the University of Manchester. “The systematic shift in R(K) in the central q² region compared to the 2021 result stems from an improved understanding of misidentified hadronic backgrounds to electrons, due to an underestimation of such backgrounds and the description of the distribution of these components in the fit. New datasets will allow us to further research this interesting topic, along with other key measurements relevant to the flavour anomalies.”

The search goes on

The flavour anomalies are a set of discrepancies observed over the past several years in processes involving b → s and b → c quark transitions. Among the former is the parameter P₅′ based on angular distributions of the decay products of B-meson decays. Although these remain unaffected by the new LHCb result, tests of LFU via R(K)-type measurements are theoretically cleaner. On 18 October, complementing previous results by Belle, BaBar and LHCb, the LHCb collaboration made the first simultaneous measurement at a hadron collider of the parameter R(D), which compares the rates of B → Dτν and B → Dμν decays, and its counterpart R(D*). Involving b → c quark transitions, such decays proceed via the tree-level exchange of a virtual W boson. Based on Run 1 data, the new values of R(D) and R(D*) are compatible both with the current world average and with the SM prediction at 2.2 σ and 2.3 σ, respectively.

“Earlier LHCb indications of anomalies with lepton flavour universality triggered immense excitement, not least because possible new-physics explanations resonated with other hints of deviations from the SM,” says CERN theorist Michelangelo Mangano. “That such anomalies could have been real shows how little we know about the deep origin of flavour symmetries and their relation with the Higgs, and highlights the key role of experimental guidance. Theoretical efforts to interpret the anomalies explored novel avenues, exposing a myriad of unanticipated phenomena possibly emerging at distances shorter than those so far described by the SM. The latest LHCb findings take nothing away from our mission to push further the boundary of our knowledge, and the search for anomalies goes on!”