$b \to c l \bar{\nu}$ decays at LHCb

TL;DR

This paper reports on recent LHCb measurements of semileptonic $b$-hadron decays, testing Lepton Flavour Universality and studying $D^{*+}$ polarisation, revealing tensions with the Standard Model in certain ratios.

Contribution

It presents new measurements of LFU ratios and $D^{*+}$ polarisation in $b$-hadron decays, updating world averages and testing Standard Model predictions.

Findings

LFU ratios show over three sigma tension with SM

$D^{*+}$ polarisation agrees with SM

Updated world averages highlight potential new physics

Abstract

This report introduces two recent measurements of semileptonic $b$-hadron decays at the LHCb experiment, including a test of Lepton Flavour Universality (LFU) using $\bar{B}^0 \to D^{(*)+} l^- \bar{\nu}_l$ decays where $l\in\{\mu,\tau\}$, and a study of the $D^{*+}$ longitudinal polarisation in the $\bar{B}^0 \to D^{*+} \tau^- \bar{\nu}_\tau$ decay. With the inclusion of the new results of the LFU ratios, the world average on $R(D)$ and $R(D^*)$, still shows a tension over three standard deviations from the SM prediction, while the measured $D^{*+}$ longitudinal polarisation is compatible with the SM value.