Evidence of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>B</mml:mi><mml:mo mathvariant="bold">+</mml:mo></mml:msup><mml:mo>→</mml:mo><mml:msup><mml:mi>τ</mml:mi><mml:mo mathvariant="bold">+</mml:mo></mml:msup><mml:mi>ν</mml:mi></mml:math>decays with hadronic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>B</mml:mi></mml:math>tags

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