Measurement of R($D^{*}$) with hadronic $\tau$ decays

Lepton universality, described in the Standard Model (SM), predicts equal coupling between gauge bosons and the three lepton families. SM extensions give additional interactions, implying in some cases a stronger coupling with the third generation of leptons. Semileptonic decays of b-hadrons provide a sensitive probe to such New Physics effects. The presence of additional charged Higgs bosons, required by such SM extensions, can have significant effect on the semileptonic decay rate of $B^0\rightarrow D^*\tau\nu$. A probe of new physics effects is the measurement of the quantities: $R(D^*)=\frac{\boldsymbol{B}(\bar{B}^0\rightarrow D^{*+}\tau ^-\bar{\nu}_\tau)}{\boldsymbol{B}(\bar{B}^0\rightarrow D^{*+}\mu^-\bar{\nu}_\mu)}$ and $R(D)=\frac{\boldsymbol{B}(\bar{B}^0\rightarrow D^{+}\tau ^-\bar{\nu}_\tau)}{\boldsymbol{B}(\bar{B}^0\rightarrow D^{+}\mu^-\bar{\nu}_\mu)}$. The combination of experimental measurements performed by BaBar, Belle and LHCb observing the channel where the τ decays in leptons, gives a deviation from the SM prediction of about 4 σ. It is therefore important to perform additional measurements in this sector in order to improve the precision and confirm or disprove this deviation. Another possibility is to perform this measurement using the channel where the semileptonic τ decays in 3 pions. This in LHCb allows to have e better reconstruction of vertices and other kinematic variables. Results obtained by LHCb on B 0 → D*τν decays, where the τ decays hadronically, are reported.