Implications of new evidence for lepton-universality violation in $b\to s{\ell }^{+}{\ell }^{-}$ decays

Abstract

Motivated by renewed evidence for new physics in $b\to s\ell \ell$ transitions in the form of LHCb's new measurements of theoretically clean lepton-universality ratios and the purely leptonic $B_s\to {\mu }^{+}{\mu }^{-}$ decay, we quantify the combined level of discrepancy with the Standard Model and fit values of short-distance Wilson coefficients. A combination of the clean observables $R_K$, $R_{K^{\ast }}$, and $B_s\to \mu \mu$ alone results in a discrepancy with the Standard Model at $4.0\sigma$, up from $3.5\sigma$ in 2017. One-parameter scenarios with purely left-handed or with purely axial coupling to muons fit the data well and result in a $\sim 5\sigma$ pull from the Standard Model. In a two-parameter fit of %$C_9$ and $C_{10}$, new-physics contributions with both vector and axial-vector couplings to muons the allowed region is much more restricted than in 2017, principally due to the much more precise result on $B_s\to {\mu }^{+}{\mu }^{-}$, which probes the axial coupling to muons.Including angular observables data restricts the allowed region further.A by-product of our analysis is an updated average of $\text{BR}(B_s\to {\mu }^{+}{\mu }^{-})=(2.8\pm 0.3)\times 10^{-9}$.

In the Appendix, we update the clean fits by considering the latest measurements of the branching fraction ratios $R_{K_s^0}$ and $R_{K^{\ast +}}$ by the LHCb Collaboration and show that two one-parameter scenarios and a two-parameter scenario fit the data well and result in a pull from the SM crossing the 5.0$\sigma$ threshold

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