Measurement of the ratio of branching fractions $\mathcal{B}(B^0 \to D^{*-}\tau^{+}\nu_\tau)/\mathcal{B}(B^0 \to D^{*-}\mu^{+}\nu_\mu)$ with $\tau$ three-prong decays

The prospects of a measurement of the branching fraction ratio $\mathcal{R}(D^*) \equiv \mathcal{B}(B^0 \to D^{*−} \tau^+ \nu_{\tau})/ \mathcal{B}(B^0 \to D^{*−} \mu^+ \nu_{\mu})$ is performed by using a data sample, corresponding to $3 \text{fb}^{−1}$ integrated luminosity, collected by LHCb at center-of-mass energies of 7 and 8 TeV during the LHC Run 1. The $\tau$ lepton is reconstructed in the hadronic decay with three charged pions. Backgrounds from prompt $B^0 \to D^* 3 \pi (X)$ decays are rejected by requiring that the τ decay vertex lies downstream of the $B^0$ decay vertex. Other physical backgrounds due to $B$ decays to double charmed hadrons are suppressed by partial reconstruction techniques, isolation criteria , and the dynamics of the $3 \pi$ system. LHCb result based on the Run1 statistics will be published soon with a statistical precision of 6.7%, the best achieved for a single measurement so far. The systematic error will be likely larger than this but can be reduced by joint efforts of the LHCb, BELLE and BES collaborations. This analysis will also enable the search for SM deviations in the event distributions, in addition to the event yield, thanks to its unique capability to select high statistics (a few thousands events) highly enriched (50%) in semi-tauonic decays.LHCb will use the exact same method to perform the measurement of all other B hadrons semitauonic decays, including those coming from $\Lambda^0_b$ and $B^+_c$ hadrons.