R(D*) and other tauonic decays at LHCb

The measurement of the branching fraction ratio $\mathcal{R}(D^{*}) \equiv \mathcal{B}(\overline{B}^0 \to D^{*+}\tau^{-}\overline{\nu}_{\tau})/\mathcal{B}(\overline{B}^0 \to D^{*+}\mu^{-}\overline{\nu}_{\mu})$ is performed using a sample of proton-proton collision data corresponding to 3.0 $\text{fb}^{-1}$ of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The chosen tau decay mode is $\tau^{-} \to \mu^{-}\overline{\nu}_{\mu}\nu_{\tau}$. A multidimensional fit to kinematic distributions of the candidate $\overline{B}^0$ decays gives $\mathcal{R}(D^{*}) = 0.336 \pm 0.027(stat) \pm 0.030 (syst)$. This result, which is the first measurement of this quantity at a hadron collider, is $2.1$ standard deviations larger than the value expected from lepton universality in the Standard Model. An ongoing analysis aiming at measuring the $\mathcal{B}(\overline{B}^0 \to D^{*+}\tau^{-}\overline{\nu}_{\tau})$ branching fraction through three-prong tau decays is expected to give competitive results with the published one.