  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 $\tau^{-} \to \mu^{-}\overline{\nu}_{\mu}\nu_{\tau}$. 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 $\overline{B}^0$ decays gives
$\mathcal{R}(D^{*}) = 0.336 \pm 0.027(stat) \pm 0.030 (syst)$. 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.
