Constraints on lepton flavor universal and non-universal New Physics in $b\, \to\, s\, \ell^+ \ell^-$ decays: a global SMEFT survey

Abstract

The flavor-changing neutral current semileptonic decays of $B$ mesons provide an excellent platform for indirectly probing New Physics (NP) beyond the Standard Model (SM). Recent measurements of lepton flavor universality (LFU) ratios such as $R_K$, $R_{K^*}$, and $R_\phi$ are consistent with SM predictions, thereby reducing earlier hints of LFU violation. Nevertheless, notable tensions persist in individual branching fractions -- for instance $\mathcal{B}(B^+ \to K^+ \mu^+ \mu^-)$ and $\mathcal{B}(B^+ \to K^+ e^+ e^-)$, which deviate at the $4\sigma$--$5\sigma$ level in the low-$q^2$ region ($[1.1,6]$ GeV$^2$). Similarly, the angular observable $P'_5$ in $B \to K^{*} \mu^+ \mu^-$ shows a $3.3\sigma$ deviation, while $\mathcal{B}(B_s \to \phi \mu^+ \mu^-)$ departs from the SM by $3.6\sigma$. Although such discrepancies could in principle originate from underestimated hadronic effects, they may also indicate possible NP contributions. In this work, we perform a global fit to the latest $b \to s \ell^+ \ell^-$ data within the framework of dimension-6 SMEFT operators. Our analysis systematically incorporates hadronic uncertainties from form factors, non-factorizable corrections, and input parameters, thereby providing a balanced assessment of the NP interpretation. We allow for NP contributions not only in $b \to s \mu^+ \mu^-$ transitions but also in $b \to s e^+ e^-$ channels, thus testing both lepton flavor universal (LFU) and lepton flavor universality violating (LFUV) NP scenarios. The interplay between these possibilities yields distinctive signatures in branching fractions, angular observables, and $\Delta$-observables, offering a comprehensive view of the current flavor anomalies.