On lepton non-universality in exclusive $b\to s \ell\ell$ decays

Abstract

The LHCb measurements of certain ratios of decay modes testing lepton flavour non-universality might open an exciting world of new physics beyond the standard model. The latest LHCb measurements of R_K* offer some new insight beyond the previous measurement of R_K. We work out the present significance for non-universality, and argue that claims of 5 sigma deviations from the SM based on all present b -> s l+l- data including the ratios are misleading and are at present still based on guesstimates of hadronic power corrections in the b -> s l+l- angular observables. We demonstrate that only a small part of the luminosity of 50 fb^-1 foreseen to be accumulated by the LHCb will be needed to offer soon a definite answer to the present question whether we see a very small glimpse of lepton flavour non-universal new physics or not. We also present new predictions for other ratios based on our analysis of the present measurements of the ratios R_K^(*) and analyse if they are able to differentiate between various new physics options within the effective field theory at present or in the near future.

12 pages, 2 figures. v2: Extended version with a detailed analysis of future prospects, references added. v3: Analysis on the present and future role of B_s -> mu+ mu- added