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

TL;DR

Recent measurements of lepton-universality ratios and $B_s o \mu^+\mu^-$ decay show a significant deviation from the Standard Model, suggesting potential new physics in $b o s\ell^+\ell^-$ transitions.

Contribution

This paper provides updated analysis of experimental data, quantifies the discrepancy with the Standard Model, and constrains new physics scenarios involving Wilson coefficients.

Findings

Discrepancy with SM increased to 4.0σ from 3.5σ since 2017.

Purely left-handed or axial coupling scenarios fit data well with ~5σ deviation.

Including angular observables further restricts new physics parameter space.

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 $R_K$, $R_{K^*}$, 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 $\text{BR}(B_s \to \mu^+ \mu^-) = (2.8\pm 0.3) \times 10^{-9}$.