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 $B\to D^{\ast }\tau nu$, are highly sensitive to New Physics particles which preferentially couple to the 3rd generation.A measurement of $R(D^{\ast -})$ using 3-prong $\tau$ decays performed by LHCb is reported.