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

Abstract

The confirmation of excess in $R_{D^{\ast }}$ 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 $R_D$, $R_{D^{\ast }}$ and $q^2$ spectra, were identfied previously. In this work, we try to find angular observables in $\bar{B}\to D^{\ast }\tau \bar{\nu }$ which enable us to distinguish between these new physics operators. We find that the $D^{\ast }$ polarization fraction $f_L(q^2)$ is a good discriminant of scalar and tensor new physics operators. The change in $⟨f_L(q^2)⟩$, induced by scalar and tensor operators, is about three times larger than the expected uncertainty in the upcoming Belle measurement.

Comments: 8 pages, 2 figures; published in Physical Review D