A search for the decays B^+to e^+ ν_e and B^+to μ^+ ν_μ using hadronic-tag reconstruction

We report on a search for the rare decay modes $B^+\to e^+ \nu_e$ and $B^+\to \mu^+ \nu_\mu$ with data collected from the BaBar detector at the PEP-II $e^+e^-$ storage ring. This search utilizes a new technique in which we fully reconstruct the accompanying $B^-$ in $\Upsilon(4S)\righarrow B^+ B^-$ events, and look for a mono-energetic lepton in the $B^+$ rest frame. No signal candidates are observed in either of the channels, consistent with the expected background, in a data sample of approximately 229 million $B\bar{B}$ pairs. The branching-fraction upper limits are set at $\mathcal{B}(B^+\to e^+\nu_{e})<7.9 \times 10^{-6}$ and $\mathcal{B}(B^+\to\mu^+\nu_{\mu})<6.2 \times 10^{-6}$ at the 90% confidence level.