Test of lepton flavour universality with semi-tauonic decays of $b$-hadrons at LHCb

Lepton Flavour Universality (LFU) is enforced in the Standard Model by construction. Any violation of LFU would be a clear sign of new physics. Existing hints of non universality are already present. The semitauonic decays in particular, are sensitive to contributions from non-Standard-Model particles that preferentially couple to the third generation of fermions, such as Higgs-like charged scalars. The LHCb experiment has measured the branching frac- tion ratio R ( D ∗ ) ≡ B ( ̄ B 0 → D ∗ + τ − ̄ ν τ ) / B ( ̄ B 0 → D ∗ + μ − ̄ ν μ ) using a sample of proton-proton collision data corresponding to an integrated luminosity of 3 f b − 1 recorded until 2012. Only muonic tau decays, τ − → μ − ̄ ν μ ν τ , are considered. The ratio is measured to be R ( D ∗ ) = 0 . 336 ± 0 . 027 ( stat ) ± 0 . 030 ( syst ) , which is 2.1 standard deviations larger than the value expected from lepton universality in the Standard Model.