Measurement of the ratio of branching fractions $\mathcal{B}({\bar{B}}^0\to D^{\ast +}{\tau }^{-}{\bar{\nu }}_{\tau })\mathrm{/}\mathcal{B}({\bar{B}}^0\to D^{\ast +}{\mu }^{-}{\bar{\nu }}_{\mu })$

The branching fraction ratio $$\mathcal{R}$(D^{}) \equiv $\mathcal{B}$(\overline{B}^0 \to D^{+}\tau^{-}\overline{\nu}{\tau})/$\mathcal{B}$(\overline{B}^0 \to D^{*+}\mu^{-}\overline{\nu}{\mu})$ is measured using a sample of proton-proton collision data corresponding to 3.0\invfb of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode ${\tau }^{-}\to {\mu }^{-}{\bar{\nu }}_{\mu }{\nu }_{\tau }$. The semitauonic decay is sensitive to contributions from non-Standard-Model particles that preferentially couple to the third generation of fermions, in particular Higgs-like charged scalars. A multidimensional fit to kinematic distributions of the candidate ${\bar{B}}^0$ decays gives $\mathcal{R}(D^{\ast })=0.336\pm 0.027(stat)\pm 0.030(syst)$. This result, which is the first measurement of this quantity at a hadron collider, is 2.1 standard deviations larger than the value expected from lepton universality in the Standard Model.