New physics effects in semileptonic $\bar{B_s} \to K^{*+}(\to Kπ) \ell^- \barν_\ell$ decay

TL;DR

This work studies new physics in the $b \to u \ell \nu$ sector by analyzing the semileptonic decay $\bar{B_s} \to K^{*+}(\to K\pi) \ell^- \bar{\nu}_\ell$ within a model-independent effective-field-theory framework. It introduces a general low-energy Hamiltonian with vector, scalar, and tensor NP operators $O_{V_L}, O_{V_R}, O_{S_L}, O_{S_R}, O_T$ and corresponding Wilson coefficients $C_i$, constraining them with existing $B$-decay data via a $\chi^2$ fit and computing NP predictions for differential rates and angular observables using hadronic form factors from lattice and LCSR inputs. The analysis finds that certain two-operator combinations, notably $S17$ with $(C_A, C_P)=(C_{V_L}=-C_{V_R}, C_{S_L}=-C_{S_R})$, can modestly improve the SM fit and yield distinctive signatures in $d\mathcal{B}/dq^2$, $A_{FB}$, and the normalized angular observables $\tilde{J}_i$. The results indicate that upcoming Run 3 LHCb and Belle II measurements of $\bar{B_s} \to K^{*+}(\to K\pi) \ell^- \bar{\nu}_\ell$ could decisively test these NP patterns and refine the allowed parameter space, with potential SMEFT interpretations for a more complete theoretical framework.

Abstract

We analyze the new physics effects in semileptonic $\bar{B_s} \to K^{*+}(\to Kπ) \ell^- \barν_\ell$ decay induced by the $b \to u \ell ν_{\ell}$ quark level transition. We consider the vector, scalar and tensor new physics Lorentz structures in addition to the SM in effective field theory approach. Constraints on new physics parameters are obtained from experimental observations of both semileptonic and leptonic decays of $B$ mesons, which are governed by the underlyng $b \to u \ell ν_\ell$ transition. We explore the new physics effects in differential branching fraction, lepton forward-backward asymmetry, fraction of longitudinal polarization of $K^*$ meson and normalized angular observables in $\bar{B_s} \to K^{*+}(\to Kπ) \ell^- \barν_\ell$ decay.

Figures (2)

• Figure 1: Contours for the allowed parameter space of $68\%$ (light orange area) and $95\%$ (orange area) for new Physics WCs obtained from our fit using the available measurements.
• Figure 2: The kinematics of the semileptonic decay $\bar{B_s} \to K^{*+}(\to K\pi) \ell^- \bar{\nu}_\ell$