Resolution of $R_D$/$R_{D^*}$ puzzle

TL;DR

The paper discusses the $R_D$/$R_{D^*}$ puzzle, a deviation from Standard Model predictions in flavor physics, and proposes specific observables in $B ightarrow D^* auar{ u}$ decay to identify the underlying new physics Lorentz structure.

Contribution

It introduces polarization fractions and angular asymmetries as tools to distinguish different new physics models explaining the $R_D$/$R_{D^*}$ puzzle.

Findings

Polarization fractions can differentiate new physics scenarios.

Angular asymmetries provide unique signatures of Lorentz structures.

Measurement of these observables can identify the nature of new physics.

Abstract

One of the exciting results in flavor physics in recent times is the $R_D$/$R_{D^*}$ puzzle. The measurements of these flavor ratios performed by the B-factory experiments, BaBar and Belle, and the LHCb experiment are about $4\sigma$ away from the Standard Model expectation. These measurements indicate that the mechanism of $b\rightarrow c\tau\bar{\nu}$ decay is not identical to that of $b\rightarrow c(\mu/e)\bar{\nu}$. This charge lepton universality violation is particularly intriguing because these decays occur at tree level in the Standard Model. In particular, we expect a moderately large new physics contribution to $b\rightarrow c\tau\bar{\nu}$. The different types of new physics amplitudes, which can explain the $R_D$/$R_{D^*}$ puzzle, have been identified previously. In this letter, we show that the polarization fractions of $\tau$ and $D^*$ and the angular asymmetries $A_{FB}$ and $A_{LT}$ in $B\rightarrow D^*\tau\bar{\nu}$ decay have the capability to uniquely identify the Lorentz structure of the new physics. A measurement of these four observables will lead to such an identification.