Binwise exploration of vector couplings in $B_s \to D_s^{(*)} τ\bar ν_τ$ decays

TL;DR

The paper addresses potential lepton flavor universality violations in charged-current $b\to c\tau\bar\nu$ transitions by performing a model-independent, binwise $q^2$ analysis of $B_s\to D_s^{(*)}\tau\bar\nu_\tau$ decays. It extends the SM with axial/vector NP operators parameterized by Wilson coefficients $V_L$ and $V_R$, and conducts a global $\chi^2$ fit to $b\to(u,c)\tau\bar\nu_\tau$ data across six NP scenarios, under two data-inclusion schemes. Using lattice QCD form factors, the authors compute binwise branching ratios, lepton-nonuniversality ratios, forward-backward asymmetries, and polarization observables for $B_s\to D_s^{(*)}\tau\bar\nu_\tau$ in four $q^2$ bins, finding that the 2DS-III and 4DS-I scenarios yield the most pronounced deviations from the SM in several observables. The results highlight the sensitivity of the $\chi_i^{D_s^{(*)}}$ observables to NP-induced shape distortions and motivate dedicated LFU tests in $B_s$ decays at future experiments to constrain or reveal new physics.

Abstract

Recent results from the LHCb experiment have confirmed that lepton flavor universality is upheld in flavor-changing neutral current processes, such as $B \to K^{(*)} l^+ l^-$. However, discrepancies remain in the charged current sector, raising questions about the universality of lepton flavors in these processes. To explore this issue, we investigate the decays $B_s \to D_s^{(*)} τ\bar ν_τ$ in different $q^2$ bins, which involve the $b \to c τ\bar ν_τ$ transition. We employ a model-independent approach to analyze potential new physics by fitting both real and complex (axial)vector coefficients to the existing $b \to (u, c) τ\bar ν_τ$ data. Our analysis enables us to calculate the branching ratios and angular distributions for the $B_s \to D_s^{(*)} τ\bar ν_τ$ decays in four different $q^2$ bins. Additionally, we evaluate whether lepton flavor universality is maintained in these charged current decays or if deviations suggest the presence of new physics.

Figures (7)

• Figure 1: $\chi^2$ analysis plots for new physics scenarios in Case A, including axial(vector) coefficients, depicting the fit quality and confidence regions.
• Figure 2: Same as Fig. \ref{['fig:CA-contours']} for the Case B.
• Figure 3: The $q^2$ binwise predictions for the branching ratios of $B_s \to D_s \tau \bar{\nu}_\tau$ (left panel) and $B_s \to D_s^* \tau \bar{\nu}_\tau$ (right panel) decay processes are shown for all new physics scenarios. The red circle represents the SM central value prediction, and the red error lines indicate the $1\sigma$ theoretical uncertainties. Predictions from the various new physics scenarios are presented in different colors.
• Figure 4: Same as Fig. \ref{['fig:BR']} for $R_{D_s}$ (left panel) and $R_{D_s^*}$ (right panel).
• Figure 5: Same as Fig. \ref{['fig:BR']} for $A_{FB}^{D_s}$ (top left panel), $A_{FB}^{D_s^*}$ (top right panel), $\chi_1^{D_s}$ (middle left panel), $\chi_1^{D_s^*}$ (middle right panel), $\chi_2^{D_s}$ (bottom left panel) and $\chi_2^{D_s^*}$ (bottom right panel).