Evidence of $B^{\mathbf{+}}\to {\tau }^{\mathbf{+}}\nu$ decays with hadronic $B$ tags

We present a search for the decay $B^{+}\to {\tau }^{+}\nu$ using $467.8\times {10}^6$ $B\overline{B}$ pairs collected at the ${\rm Y}(4S)$ resonance with the BABAR detector at the SLAC PEP-II $B$-Factory. We select a sample of events with one completely reconstructed $B^{-}$ in the hadronic decay mode ($B^{-}\to D^{(*)0}{X}^{-}$ and $B^{-}\to J/\psi X^{-}$). We examine the rest of the event to search for a $B^{+}\to {\tau }^{+}\nu$ decay. We identify the ${\tau }^{+}$ lepton in the following modes: ${\tau }^{+}\to e^{+}\nu \overline{\nu }$, ${\tau }^{+}\to {\mu }^{+}\nu \overline{\nu }$, ${\tau }^{+}\to {\pi }^{+}\overline{\nu }$ and ${\tau }^{+}\to {\rho }^{+}\overline{\nu }$. We find an excess of events with respect to the expected background, which excludes the null signal hypothesis at the level of $3.8\sigma$ (including systematic uncertainties) and corresponds to a branching fraction value of $\mathcal{B}(B^{+}\to {\tau }^{+}\nu )=({1.83}_{-0.49}^{+0.53}(\mathrm{stat})\pm 0.24(\mathrm{syst}))\times {10}^{-4}$.

Figure 1

Fit to the $m_{\mathrm{ES}}$ distribution in data. Dots are data, the upper curve is the global fit result and the lower curve represents the fitted combinatorial and continuum background.

Figure 2

$E_{\mathrm{extra}}$ distribution in data (points with error bars) with all selection requirements applied and fit results overlaid. The hatched histogram is the background and the dashed component is the best-fit signal excess distribution. Plot (a) shows all $\tau$ decay modes fitted simultaneously. Lower plots show the projection of the simultaneous fit result on the four analyzed $\tau$ decay modes: (b) ${\tau }^{+}\to e^{+}\nu \overline{\nu }$, (c) ${\tau }^{+}\to {\mu }^{+}\nu \overline{\nu }$, (d) ${\tau }^{+}\to {\pi }^{+}\nu$, (e) ${\tau }^{+}\to {\rho }^{+}\nu$.

Figure 3

$E_{\mathrm{extra}}$ distribution for double tagged events. The “signal” $B$ is reconstructed in hadronic decays (left plot) or semileptonic decays (right plot). Points are data and boxes are MC simulation.

Figure 4

Top plot: Comparison between the measured $\mathcal{B}(B^{+}\to {\tau }^{+}\nu )$ branching fraction (horizontal band) with the prediction of the 2HDM as a function of $\tan \beta /m_{H^{+}}$, using exclusive (red/light gray) or inclusive (blue/dark gray) $|V_{ub}|$ measurement. Bottom plots: 90% and 99% C.L. exclusion regions in the $(m_{H^{+}},\tan \beta )$ plane using the exclusive (left) and inclusive (right) measurements of $|V_{ub}|$.