# Model-independent method for measuring the angular coefficients of $B^0 \to D^{*-} \tau^+ \nu_{\tau}$ decays

## TL;DR

This paper introduces a new model-independent method to measure angular coefficients in $B^0 o D^{*-} au^+
u_{ au}$ decays, overcoming biases caused by neutrino loss and enabling precise analysis at LHCb and Belle II.

## Contribution

A novel, unbiased, model-independent approach for measuring angular coefficients in $B^0$ decays that accounts for resolution and acceptance effects using multidimensional fits.

## Key findings

- Method successfully models angular functions and background in a multidimensional fit.
- Sensitivity estimates provided for LHCb and Belle II experiments.
- Systematic uncertainties, especially in $R(D^{*})$ extrapolation, are discussed.

## Abstract

Reconstruction of the $B^0 \to D^{*-} \tau^+ \nu_{\tau}$ angular distribution is complicated by the strongly-biasing effect of losing the neutrino information from both the $B$ and $\tau$ decays. In this work, a novel method for making unbiased measurements of the angular coefficients while preserving the model independence of the angular technique is demonstrated. The twelve angular functions that describe the signal decay, in addition to background terms, are modelled in a multidimensional fit, using template probability density functions that encapsulate all resolution and acceptance effects. Sensitivities at the LHCb and Belle II experiments are estimated, and sources of systematic uncertainty are discussed, notably in the extrapolation to a measurement of $R(D^{*})$.