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 $R_K$, $R_{K^{\ast }}$, and $R_{\varphi }$ 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 $P_5^{\prime }$ in $B\to K^{\ast }{\mu }^{+}{\mu }^{-}$ shows a $3.3\sigma$ deviation, while $\mathcal{B}(B_s\to \varphi {\mu }^{+}{\mu }^{-})$ 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.

Comments: 31 pages, 10 figures, 24 tables , matches with the published version