A Complete Framework for Tau Polarimetry in $B\to D^{(*)}\tau\nu$ Decays

TL;DR

This paper develops a comprehensive method to analyze tau polarization in B meson decays, enabling detailed exploration of potential new physics effects in these processes.

Contribution

It introduces a complete framework for extracting tau polarization asymmetries in B decays, enhancing the ability to probe new physics beyond the Standard Model.

Findings

Seven tau asymmetries can be extracted from decay kinematics.

At Belle II, asymmetries could be measured with percent level precision.

The method enhances sensitivity to new physics in $b o c\tau\nu$ transitions.

Abstract

The meson decays $B\to D\tau\nu$ and $B\to D^* \tau \nu$ are sensitive probes of the $b\to c\tau\nu$ transition. In this work we present a complete framework to obtain the maximum information on the physics of $B\to D^{(*)}\tau\nu$ with polarized $\tau$ leptons and unpolarized $D^{(*)}$ mesons. Focusing on the hadronic decays $\tau\to \pi\nu$ and $\tau\to\rho\nu$, we show how to extract seven $\tau$ asymmetries from a fully differential analysis of the final-state kinematics. At Belle II with $50~\text{ab}^{-1}$ of data, these asymmetries could potentially be measured with percent level statistical uncertainty. This would open a new window into possible new physics contributions in $b\to c\tau\nu$ and would allow us to decipher its Lorentz and gauge structure.