Diagnosing New Physics in $b \to c \, \tau \, \nu_\tau$ decays in the light of the recent BaBar result

TL;DR

This paper investigates potential new physics explanations for recent anomalies in B meson decays involving tau leptons, analyzing how different new physics operators could be distinguished through various observables.

Contribution

It introduces an effective Lagrangian framework to interpret the BaBar anomalies and proposes specific observables to identify the nature of possible new physics couplings.

Findings

Forward-backward asymmetries can differentiate new physics operators.

Tau and D* polarization fractions provide insights into the couplings.

Results suggest certain new physics scenarios are compatible with observed deviations.

Abstract

The BaBar Collaboration has recently reported the measurement of the ratio of the branching fractions of $\bar{B} \to D(D^{*}) \tau^{-} \bar{\nu_\tau}$ to $\bar{B} \to D(D^{*}) \ell^{-} \bar{\nu_\ell}$ which deviates from the Standard Model prediction by 2$\sigma$(2.7$\sigma$). This deviation goes up to 3.4$\sigma$ level when the two measurements in the $D$ and $D^*$ modes are taken together and could indicate new physics. Using an effective Lagrangian for the new physics we study the implication of these results and calculate other observables that can shed light on the nature of the new physics. We show that the measurements of the forward-backward asymmetries and the $\tau$ and $D^*$ polarization fractions can distinguish among the various couplings of the new physics operators.