# New Physics in $b\to c \tau \nu$: Impact of Polarisation Observables and $B_c\to\tau\nu$

## TL;DR

This paper investigates the anomalies in B-meson decays involving tau leptons, analyzing how polarization observables and B_c decay constraints influence new physics interpretations and predictions.

## Contribution

It performs a comprehensive fit of b→cτν data considering polarization and B_c decay limits, highlighting the high resolving power of polarization observables and the correlation between R(D(*) ) and R(Λ_c).

## Key findings

- Polarization observables have high model-resolving power.
- Enhancement in R(D(*) ) implies enhancement in R(Λ_c).
- A sum-rule links R(D(*) ) and R(Λ_c) regardless of the scenario.

## Abstract

The experimental values of the lepton-flavour-universality tests ${\cal R}(D)$ and ${\cal R}(D^*)$ show a tension of about $3.1\sigma$ with their Standard Model prediction. Motivated by this tension, we perform a fit of the $b\to c\tau\nu$ data. We consider one-particle scenarios imposing consecutive limits on $\text{BR}(B_c\to \tau\nu_\tau)$, and analyse how these limits affect the fits. We include the polarisation observables available to date and predict those that are still to be measured, and conclude that they have a high model-resolving power. For each scenario we also predict ${\cal R}(\Lambda_c)$, observing that an enhancement of ${\cal R}(D^{(*)})$ implies an enhancement of ${\cal R}(\Lambda_c)$ in any scenario. We trace back this enhancement to a sum-rule valid irrespective of the scenario used to fit ${\cal R}(D^{(*)})$.