Semileptonic $B \to D^*$ decays from light to $\tau$ leptons: the extraction of the form factor $F_2$ from data

TL;DR

This paper extends the analysis of semileptonic B to D* decays to include tau leptons, extracting form factors and comparing them with lattice QCD, revealing tensions in the Standard Model predictions.

Contribution

It introduces a method to analyze B to D* tau decays without relying on |V_cb|, and provides a new determination of |V_cb| from decay data.

Findings

Difficulty in simultaneously fitting R(D*) and other decay observables within the SM.

Extraction of the form factor F_2 from data independent of |V_cb|.

A new estimate of |V_cb| from total decay rate.

Abstract

We extend the Standard Model (SM) analysis of Ref. [1], which was limited to light leptons in the final state, to the semileptonic $B \to D^* \tau \nu_\tau$ decay. By using quantities that can be analised without the knowledge of $\vert V_{cb}\vert$, we derive important information about the helicity amplitudes and the hadronic form factors that can be compared with the predictions of lattice QCD calculations. In particular, there is a difficulty in reproducing simultaneously the experimental values of $R(D^*)$ and of other quantities relevant for the semitauonic decays within the SM. As a byproduct of our analysis, we also present a determination of $\vert V_{cb}\vert$ from the total decay rate.