Observation of $B^0\to D^{*-}{\tau }^{+}{\nu }_{\tau }$ Decay at Belle

We report an observation of the decay $B^0\to D^{*-}{\tau }^{+}{\nu }_{\tau }$ in a data sample containing $535\times {10}^6$ $B\overline{B}$ pairs collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We find a signal with a significance of $5.2\sigma$ and measure the branching fraction $\mathcal{B}(B^0\to D^{*-}{\tau }^{+}{\nu }_{\tau })=\mathbf{(}{2.02}_{-0.37}^{+0.40}(\mathrm{stat})\pm 0.37(\mathrm{syst})\mathbf{)}\%$. This is the first observation of an exclusive $B$ decay with a $b\to c\tau {\nu }_{\tau }$ transition.

Figure 1

$M_{\mathrm{tag}}$ and $\Delta E_{\mathrm{tag}}$ distributions for the $B^0\to D^{*-}{\pi }^{+}$ control sample from data (points with error bars) and MC (histograms).

Figure 2

$X_{\mathrm{mis}}$ and $E_{\mathrm{vis}}$ distributions (normalized to unity) after the $B_{\mathrm{tag}}$ selection for signal (blank) and background (shaded) for the ${\tau }^{+}\to e^{+}{\nu }_e{\overline{\nu }}_{\tau }$ mode in the region $M_{\mathrm{tag}}>5.27\mathrm{GeV}/c^2$. The background components, from top to bottom: $B^0\to D^{*-}{e}^{+}{\nu }_e$, $B\to D^{**}{e}^{+}{\nu }_e$, and other $B$ decays. The contribution from $q\overline{q}$ continuum is negligible.

Figure 3

$M_{\mathrm{tag}}$ distribution for the combined data sample. The histogram represents expected background scaled to the data luminosity. The solid curve shows the result of the fit. The dotted and dashed curves indicate, respectively, the fitted background and the component described by the ARGUS function.