Lepton-flavour universality tests with semi-leptonic B decays at LHCb

In the Standard Model, the three charged lepton are identical copies of each other, apart from mass differences. Experimental tests of this feature in decays of semileptonic b hadrons, such as \(B\rightarrow D^* \tau\ nu\), are highly sensitive to New Physics particles which preferentially couple to the 3rd generation.A measurement of \(R(D^{*-})\) using 3-prong \(\tau\) decays performed by LHCb is reported.