Searches for the decaysB0→l±τ∓andB+→l+ν(l=e,μ) using hadronic tag reconstruction

We present searches for the leptonic decays ${B}^{+}\ensuremath{\rightarrow}{\ensuremath{\ell}}^{+}\ensuremath{\nu}$ and the lepton flavor violating decays ${B}^{0}\ensuremath{\rightarrow}{\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\tau}}^{\ensuremath{\mp}}$, where $\ensuremath{\ell}=e$, $\ensuremath{\mu}$, with data collected by the BABAR experiment at SLAC. This search demonstrates a novel technique in which we fully reconstruct the accompanying $\overline{B}$ in $\ensuremath{\Upsilon}(4S)\ensuremath{\rightarrow}B\overline{B}$ events, and look for a monoenergetic lepton from the signal $B$ decay. The signal yield is extracted from a fit to the signal lepton candidate momentum distribution in the signal $B$ rest frame. Using a data sample of approximately $378\ifmmode\times\else\texttimes\fi{}{10}^{6}$ $B\overline{B}$ pairs ($342\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$), we find no evidence of signal in any of the decay modes. Branching fraction upper limits of $\mathcal{B}({B}^{+}\ensuremath{\rightarrow}{e}^{+}\ensuremath{\nu})l5.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, $\mathcal{B}({B}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}\ensuremath{\nu})l5.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, $\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\tau}}^{\ensuremath{-}})l2.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ and $\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}})l2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$, are obtained at 90% confidence level.