A general effective field theory description of b → sℓ+ℓ− lepton universality ratios

Abstract

We construct an expression for a general lepton flavour universality (LFU) ratio, RX, in b→sℓ+ℓ− decays in terms of a series of hadronic quantities which can be treated as nuisance parameters. This expression allows to include any LFU ratio in global fits of b→sℓ+ℓ− short-distance parameters, even in the absence of a precise knowledge of the corresponding hadronic structure. The absence of sizeable LFU violation and the approximate left-handed structure of the Standard Model amplitude imply that only a very limited set of hadronic parameters hamper the sensitivity of RX to a possible LFU violation of short-distance origin. A global b→sℓ+ℓ− combination is performed including the measurement of RpK for the first time, resulting in a significance of new physics of 4.2σ. In light of this, we evaluate the impact on the global significance of new physics using a set of experimentally promising non-exclusive RX measurements that LHCb can perform, and find that they can significantly increase the discovery potential of the experiment.