Implications of new evidence for lepton-universality violation in b→sℓ+ℓ− decays

Abstract

Motivated by renewed evidence for new physics in $b\ensuremath{\rightarrow}s\ensuremath{\ell}\ensuremath{\ell}$ transitions in the form of LHCb's new measurements of theoretically clean lepton-universality ratios and the purely leptonic ${B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ 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 ${R}_{K}$, ${R}_{{K}^{*}}$, and ${B}_{s}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\mu}$ alone results in a discrepancy with the Standard Model at $4.0\ensuremath{\sigma}$, up from $3.5\ensuremath{\sigma}$ in 2017. One-parameter scenarios with purely left-handed or with purely axial coupling to muons fit the data well and result in a $\ensuremath{\sim}5\ensuremath{\sigma}$ pull from the Standard Model. In a two-parameter fit of 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 ${B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, 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 $\mathrm{BR}({B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})=(2.8\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$.