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

Md Isha Ali, Utpal Chattopadhyay, Dilip Kumar Ghosh, N Rajeev

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 $R_φ$ are consistent with SM predictions, thereby reducing earlier hints of LFU violation. Nevertheless, notable tensions persist in individual branching fractions -- for instance $\mathcal{B}(B^+ \to K^+ μ^+ μ^-)$ and $\mathcal{B}(B^+ \to K^+ e^+ e^-)$, which deviate at the $4σ$--$5σ$ level in the low-$q^2$ region ($[1.1,6]$ GeV$^2$). Similarly, the angular observable $P'_5$ in $B \to K^{*} μ^+ μ^-$ shows a $3.3σ$ deviation, while $\mathcal{B}(B_s \to φμ^+ μ^-)$ departs from the SM by $3.6σ$. 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 $b \to s μ^+ μ^-$ 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 $Δ$-observables, offering a comprehensive view of the current flavor anomalies.

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 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 , , and are consistent with SM predictions, thereby reducing earlier hints of LFU violation. Nevertheless, notable tensions persist in individual branching fractions -- for instance and , which deviate at the -- level in the low- region ( GeV). Similarly, the angular observable in shows a deviation, while departs from the SM by . 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 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 transitions but also in 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 -observables, offering a comprehensive view of the current flavor anomalies.

Paper Structure

This paper contains 12 sections, 30 equations, 2 figures, 24 tables.

Table of Contents

  1. Introduction
  2. SMEFT Phenomenology
  3. Low energy effective Hamiltonian
  4. Matching of SMEFT to LEFT at tree level
  5. Global fit of SMEFT coefficients and bounds on LEFT WCs
  6. Analyzing $b \to s \mu^+ \mu^-$ and $b \to s e^+ e^-$ decay observables
  7. Sensitivity of LFUV in $\Delta$-observables
  8. Summary and Conclusion
  9. Hadronic matrix elements, differential decay distribution and $q^2$ observables in $B \to K^{(*)} \ell^+ \ell^-$ decays
  10. Fast likelihood approach Straub:2018kue
  11. Prediction of $B^+ \to K^+\, \ell^+\,\ell^-$ and $B^0 \to K^*\, \ell^+\,\ell^-$ observables in SM and in the presence of various NP scenarios at different $q^2$ bins
  12. Prediction of $B_s \to \phi \ell^+ \ell^-$ and $\Lambda_b \to \Lambda\, \ell^+\,\ell^-$ ($\ell \in e,\mu$) observables in SM and in the presence of various NP scenarios at different $q^2$ bins

Figures (2)

  • Figure 1: The likelihood contours of the global fit and several fits to subsets of observables in the plane of SMEFT coefficients (i) S9: $[C_{qe}]_{2311}=[C_{qe}]_{2322}$, $[C_{\phi q}^{(3)}]_{23}$, (ii) S10: $[C_{\ell q}^{(3)}]_{1123}=[C_{\ell q}^{(3)}]_{2223}$, $[C_{\phi q}^{(3)}]_{23}$, (iii) S18: $[C_{\ell q}^{(3)}]_{1123}$, $[C_{\ell q}^{(3)}]_{2223}$ and (iv) S42: $[C_{qe}]_{2322}$, $[C_{\phi q}^{(3)}]_{23}$.
  • Figure 10: Binned $\Delta P_{2}$ and $\Delta P'_{5}$ in SM and in the presence of the NP scenario S42.