Measurement of the branching ratio of ${\overline{B}}^0\to D^{*+}{\tau }^{-}{\overline{\nu }}_{\tau }$ relative to ${\overline{B}}^0\to D^{*+}{\ell }^{-}{\overline{\nu }}_{\ell }$ decays with a semileptonic tagging method

We report a measurement of the ratio $\mathcal{R}(D^{*})=\mathcal{B}({\overline{B}}^0\to D^{*+}{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}({\overline{B}}^0\to D^{*+}{\ell }^{-}{\overline{\nu }}_{\ell })$, where $\ell$ denotes an electron or a muon. The results are based on a data sample containing $772\times 10^{6}B\overline{B}$ pairs recorded at the $\mathrm{\Upsilon }(4S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We select a sample of $B^0{\overline{B}}^0$ pairs by reconstructing both $B$ mesons in semileptonic decays to $D^{*\mp }{\ell }^{\pm }$. We measure $\mathcal{R}(D^{*})=0.302\pm 0.030(\text{stat})\pm 0.011(\text{syst})$, which is within $1.6\sigma$ of the Standard Model theoretical expectation, where the standard deviation $\sigma$ includes systematic uncertainties. We use this measurement to constrain several scenarios of new physics in a model-independent approach.

Figure 1

The $\cos {\theta }_{B-D^{*}\ell }$ distributions for ${\overline{B}}^0\to D^{*+}{\tau }^{-}{\overline{\nu }}_{\tau }$ (solid red circles) and ${\overline{B}}^0\to D^{*+}{\ell }^{-}{\overline{\nu }}_{\ell }$ (open black circles) taken from a MC simulation.

Figure 2

The $E_{\mathrm{ECL}}$ distributions for the signal (solid red circles), the normalization (open black circles), and the background (open blue triangles) taken from MC simulation, where the $E_{\mathrm{ECL}}$ is defined as the sum of the energies of neutral clusters detected in the ECL that are not associated with reconstructed particles.

Figure 3

Projections of the fit results with data points overlaid for (left) the neural network classifier output, ${\mathcal{O}}_{NB}$, and the $E_{\mathrm{ECL}}$ distribution in (center) the signal-enhanced region, ${\mathcal{O}}_{NB}>0.8$, and (right) the normalization-enhanced region, ${\mathcal{O}}_{NB}<0.8$. The background categories are described in detail in the text, where “others” refers to predominantly $B\to X_c{D}^{*}$ decays.

Figure 4

The efficiencies for (left panel) the type-II 2HDM and (right panel) $R_2$-type leptoquark model with respect to the SM value.

Figure 5

The measured values of $\mathcal{R}(D^{*})$ for (left panel) the type-II 2HDM and (right panel) $R_2$-type leptoquark models, where central values are given as the solid (red) curves and the $1\sigma$ uncertainties are given as the shaded (yellow) regions. The theoretical predictions and their $1\sigma$ uncertainties are shown as solid (blue) curves and hatched (light blue) regions, respectively [21].

Figure 6

Background-subtracted momentum distributions of $D^{*}$ (top panels) and $\ell$ (bottom panels) in the region of ${\mathcal{O}}_{NB}>0.8$ and $E_{\mathrm{ECL}}<0.5\mathrm{GeV}$ for (left panels) the SM, (center panels) the type-II 2HDM with $\tan \beta /m_{H^{+}}=0.7{\mathrm{GeV}}^{-1}$, and (right panels) the $R_2$-type leptoquark model with $C_T=+0.36$. The points and the shaded histograms correspond to the measured and expected distributions, respectively. The expected distributions are normalized to the number of detected events.