$b\to cl\bar{\nu }$ decays at LHCb

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