Observation of the Semileptonic Decays $B\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau }$ and Evidence for $B\to D{\tau }^{-}{\overline{\nu }}_{\tau }$

We present measurements of the semileptonic decays $B^{-}\to D^0{\tau }^{-}{\overline{\nu }}_{\tau }$, $B^{-}\to D^{*0}{\tau }^{-}{\overline{\nu }}_{\tau }$, ${\overline{B}}^0\to D^{+}{\tau }^{-}{\overline{\nu }}_{\tau }$, and ${\overline{B}}^0\to D^{*+}{\tau }^{-}{\overline{\nu }}_{\tau }$, which are potentially sensitive to non-standard model amplitudes. The data sample comprises $232\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ decays collected with the BABAR detector. From a combined fit to $B^{-}$ and ${\overline{B}}^0$ channels, we obtain the branching fractions $\mathcal{B}(B\to D{\tau }^{-}{\overline{\nu }}_{\tau })=(0.86\pm 0.24\pm 0.11\pm 0.06)\%$ and $\mathcal{B}(B\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau })=(1.62\pm 0.31\pm 0.10\pm 0.05)\%$ (normalized for the ${\overline{B}}^0$), where the uncertainties are statistical, systematic, and normalization-mode-related.

Figure 1

Distributions of events and fit projections in $m_{\mathrm{miss}}^2$ for the four final states: $D^{*0}{\ell }^{-}$, $D^0{\ell }^{-}$, $D^{*+}{\ell }^{-}$, and $D^{+}{\ell }^{-}$. The normalization region $m_{\mathrm{miss}}^2\approx 0$ is shown with finer binning in the insets. The fit components are combinatorial background (white, below dashed line), charge crossfeed background (white, above dashed line), the $B\to D{\ell }^{-}{\overline{\nu }}_{\ell }$ normalization mode ($//$ hatching, yellow), the $B\to D^{*}{\ell }^{-}{\overline{\nu }}_{\ell }$ normalization mode (\\ hatching, light blue), $B\to D^{**}{\ell }^{-}{\overline{\nu }}_{\ell }$ background (dark, or blue), the $B\to D{\tau }^{-}{\overline{\nu }}_{\tau }$ signal (light gray, green), and the $B\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau }$ signal (medium gray, magenta). The fit shown incorporates the $B^{-}-{\overline{B}}^0$ constraints.