$b \to c l \barν$ decays at LHCb

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 $$\bar{B}⁰ \to D^{(*)+} l^- \bar{\nu}<em>l$$ decays where $l\in{\mu,\tau}$, and a study of the $D<sup>{*+}$ longitudinal polarisation in the $$\bar{B}⁰ \to D^{*+} \tau^- \bar{\nu}</em>\tau$$ decay. With the inclusion of the new results of the LFU ratios, the world average on $R(D)$ and $R(D<sup>*)$, still shows a tension over three standard deviations from the SM prediction, while the measured $D<sup>{*+}$ longitudinal polarisation is compatible with the SM value.