Experimental mini-review on $R(D)$ and $R(D^*)$ measurements at LHCb

The large cross section for the production of b quark pairs at the LHC and the large boost of the resulting b -hadrons, combined to the excellent vertexing capabilities of the LHCb detector, enable precise measurements of semi-tauonic decays in an hadronic environment. By using a data sample of pp collisions, corresponding to 3 fb − 1 integrated luminosity, LHCb has measured the ratio R ( D ∗ ) of branching fractions for the B 0 → D ∗− τ + ν τ and B 0 → D ∗− μ + ν μ decays. The result R ( D ∗ ) = 0 . 336 ± 0 . 027 ± 0 . 030 is in tension with the Standard Model prediction at the 2 σ level and, when combined with other measurements from the B Factories on decays with D and D ∗ mesons in the final states, increases the discrepancy with the Standard Model at the 4 σ level. In this review, the LHCb measurement is described and an outlook on other possible studies in the domain of semitauonic decays of b -hadrons is given.