Study of the measurement of the ratio $\mathcal R(D_s^*)$ at LHCb

The semileptonic $b$ hadron decays with a heavy lepton are sensitive to non Standard Model couplings between quarks and leptons. The B-Factories and LHCb have previously performed various measurements of the observable $\mathcal R(D^{(*)})$. A global average of these measurements shows a discrepancy with the Standard Model expectations, which is above 3 sigma. The status of the analysis of $\mathcal R(D_s^*)$, exploiting the large amount of $B^0_s$ mesons collected by LHCb, is presented. The study of $B^0_s$ mesons provides a crucial cross-check of the existing measures, mostly based on $B^+$ and $B^0$. The treatment of some of the most important systematics to these decays will be discussed in detail.