Tests of lepton-universality as rate ratios in b → sll transitions can be predicted very accurately in the Standard Model. The deficits with respect to expectations reported by the LHCb experiment in muon-toelectron ratios of the B → K ðÃÞ ll decay rates thus point to genuine manifestations of lepton nonuniversal new physics. In this paper, we analyze these measurements in the context of effective field theory. First, we discuss the interplay of the different operators in R K and R K à and provide predictions for R K à in the Standard Model and in new-physics scenarios that can explain R K . We also provide approximate numerical formulas for these observables in bins of interest as functions of the relevant Wilson coefficients. Secondly, we perform frequentist fits to R K and R K à . The Standard Model disagrees with these measurements at 3.7σ significance. We find excellent fits in scenarios with combinations ofwith pulls relative to the Standard Model in the region of 4σ. An important conclusion of our analysis is that a lepton-specific contribution to O 10 is important to understand the data. Under the hypothesis that new-physics couples selectively to the muons, we also present fits to other b → sμμ data with a conservative error assessment and comment on more general scenarios. Finally, we discuss new lepton universality ratios that, if new physics is the origin of the observed discrepancy, should contribute to the statistically significant discovery of new physics in the near future.

We revisit the status of the new-physics interpretations of the anomalies in semileptonic B decays in light of the new data reported by Belle on the lepton-universality ratios R D ( * ) using the semileptonic tag and on the longitudinal polarization of the D * in B → D * τν, F D * L . The preferred solutions involve new left-handed currents or tensor contributions. Interpretations with pure right-handed currents are disfavored by the LHC data, while pure scalar models are disfavored by the upper limits derived either from the LHC or from the B c lifetime. The observable F D * L also gives an important constraint leading to the exclusion of large regions of parameter space. Finally, we investigate the sensitivity of different observables to the various scenarios and conclude that a measurement of the tau polarization in the decay mode B → Dτν would effectively discriminate among them. *

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