Imprints of new physics operators in the semileptonic $B \to a_1 (1260) \ell^- \barν_\ell$ process in SMEFT approach

TL;DR

The study embeds the $b \to u \ell \nu$ transition in the Standard Model Effective Field Theory and constrains dimension-6 SMEFT operators using existing leptonic and semileptonic B decays. It then computes the full angular distribution for the exclusive channel $B \to a_1(1260) \ell \nu$ (with $a_1 \to \rho\pi$) and analyzes how scalar, vector, and tensor SMEFT operators modify the longitudinal and transverse angular coefficients, the branching ratio, forward-backward asymmetry, and the lepton-flavor universality ratio ${\cal R}_{a_1}$. The findings show notable NP sensitivity, particularly in the $\tau$ mode, with certain angular coefficients and observables exhibiting pronounced deviations from SM predictions under SMEFT scenarios; these effects are testable with current and upcoming Belle-II and LHCb data. The work provides a bridge between high-scale SMEFT operators and low-energy hadronic observables in a CKM-suppressed channel, highlighting how precise angular analyses can reveal or constrain new physics in $b \to u \ell \nu$ transitions.

Abstract

At present, there are several measurements of $B$ decays that exhibit discrepancies with the predictions of the Standard Model, and suggest the presence of new physics in $b\to s$ and $b \to c(u)$ quark level transitions. Motivated by the prospects of the ongoing high-luminosity $B$ factories, we study the exclusive $B \to a_1 (1260) \ell^- \barν_\ell$ process within the Standard Model Effective Field Theory (SMEFT) formalism, to understand the sensitivity of new physics. The new physics parameters are constrained by using the experimental branching fractions of the (semi)leptonic $B \to \ell \barν$ and $B \to (π, ρ, ω) \ell \barν$ processes (where $\ell = e, μ, τ$) which undergo $b \to u \ell \barν$ quark level transitions. We then perform a comprehensive angular analysis of the exclusive $B \to a_1 (1260) \ell^- \barν_\ell$ process in the Standard Model and in the presence of various new physics operators. We also provide the predictions and comment on various observables, such as branching ratio, forward-backward asymmetry, and the test of lepton flavor non universality of the $B \to a_1 (1260) \ell^- \barν_\ell$ channel.

Figures (8)

• Figure 1: Allowed regions for all the possible 2-D couplings from $b \to u \mu \nu$ available data. The colors distinguish various decay modes shown in the right side of each panel. The black star represents the corresponding best-fit value.
• Figure 2: Similar as Fig. \ref{['Constraints']} for $b \to u \tau \nu$ channels.
• Figure 3: The $q^2$ dependency of the angular coefficients of $B \to a_1 (\rho _{||} \pi) \mu \bar{\nu}$ decay mode in SM (cyan), $S + V$ (magenta) and $S + V + T$ (yellow).
• Figure 4: The $q^2$ dependency of the angular coefficients of $B \to a_1 (\rho _{||} \pi) \tau \bar{\nu}$ decay mode in SM (red), $S + V$ (green) and $S + V + T$ (blue).
• Figure 5: The $q^2$ dependency of the angular coefficients of $B \to a_1 (\rho _{\perp} \pi) \mu \bar{\nu}$ decay mode in SM (cyan), $S + V$ (magenta) and $S + V + T$ (yellow).