A general effective field theory description of $b\to s{l}^{+}{l}^{-}$ lepton universality ratios

Abstract

We construct an expression for a general lepton flavour universality (LFU) ratio, $R_X$, in $b\to s{l}^{+}{l}^{-}$ 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 $b\to s{l}^{+}{l}^{-}$ 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 $b\to s{l}^{+}{l}^{-}$ combination is performed including the measurement of $R_{pK}$ for the first time, resulting in a significance of new physics of $4.2\sigma$. 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.

Comments: 7 pages, 4 figures