D* polarization as a probe to discriminate new physics in B --> D* tau nubar

TL;DR

This paper investigates how D* polarization measurements in B meson decays can distinguish between different new physics scenarios explaining lepton flavor non-universality anomalies.

Contribution

It identifies D* polarization fraction as a key observable to differentiate scalar and tensor new physics operators in B decay anomalies.

Findings

D* polarization fraction effectively discriminates scalar and tensor operators.

Predicted changes in polarization are significantly larger than experimental uncertainties.

Provides a method to identify the nature of new physics in B decays.

Abstract

The confirmation of excess in $R_{D^*}$ at the LHCb is an indication of lepton flavor non-universality. Various different new physics operators and their coupling strengths, which provide a good fit to $R_D$, $R_{D^*}$ and $q^2$ spectra, were identfied previously. In this work, we try to find angular observables in $\bar{B} \to D^* \tau \bar{\nu}$ which enable us to distinguish between these new physics operators. We find that the $D^*$ polarization fraction $f_L(q^2)$ is a good discriminant of scalar and tensor new physics operators. The change in $\langle f_L(q^2) \rangle$, induced by scalar and tensor operators, is about three times larger than the expected uncertainty in the upcoming Belle measurement.