  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.
