Non-perturbative bounds for $B \to D^{(*)}\ell\nu_{\ell}$ decays and phenomenological applications

Abstract

We show how to extract the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $\vert V_{cb} \vert$ from exclusive semileptonic $B \to D^{(*)}$ decays by using the Dispersive Matrix (DM) method. It is a new approach which allows to determine in a full non-perturbative way the hadronic form factors (FFs) in the whole kinematical range, without making any assumption on their dependence on the momentum transfer. We investigate also the issue of Lepton Flavor Universality (LFU) by computing a pure theoretical estimate of the ratio $R(D^{(*)})$. Our approach is applied to the preliminary LQCD computations of the FFs, published by the FNAL/MILC [1] and the JLQCD [2] Collaborations, for the $B \to D^*$ decays and to the final ones, computed by FNAL/MILC [3], for the $B \to D$ transitions . Since the FNAL/MILC Collaborations have recently published the final results of their LQCD computations of the FFs [4] for the $B \to D^*$ case, we present also the results of our procedure after its application on these data. We find $ \vert V_{cb} \vert = (41.0 \pm 1.2) \cdot 10^{-3}$ and $\vert V_{cb} \vert = (41.3 \pm 1.7) \cdot 10^{-3}$ from $B \to D$ and $B \to D^*$ decays, respectively. These estimates are consistent within $1\sigma$ with the most recent inclusive determination $\vert V_{cb}\vert_{incl} = (42.16 \pm 0.50) \cdot 10^{-3}$ [5]. Furthermore, we obtain $R(D) = 0.289(8)$ and $R(D^*) = 0.269(8)$, which are both compatible with the latest experimental averages [6] at the $\sim$1.6$\sigma$ level.