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

We construct an expression for a general lepton flavour universality (LFU) ratio, R_X, 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 R_X to a possible LFU violation of short-distance origin. A global b→sℓ⁺ℓ⁻ combination is performed including the measurement of R_pK 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 R_X measurements that LHCb can perform, and find that they can significantly increase the discovery potential of the experiment.