Measurement of the Ratio of Branching Fractions $\mathcal{B}({\overline{B}}^0\to D^{*+}{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}({\overline{B}}^0\to D^{*+}{\mu }^{-}{\overline{\nu }}_{\mu })$

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 $3.0{\mathrm{fb}}^{-1}$ 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 $\mathcal{R}(D^{*})=0.336\pm 0.027(\mathrm{stat})\pm 0.030(\mathrm{syst})$. 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.

Figure 1

Distributions of $m_{\mathrm{miss}}^2$ (left) and $E_{\mu }^{*}$ (right) of the four $q^2$ bins of the signal data, overlaid with projections of the fit model with all normalization and shape parameters at their best-fit values. Below each panel differences between the data and fit are shown, normalized by the Poisson uncertainty in the data. The bands give the $1\sigma$ template uncertainties.