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

We present searches for the leptonic decays B{sup +} {yields} {ell}{sup +}{nu} and the lepton flavor violating decays B{sup 0} {yields} {ell}{sup {+-}}{tau}{sup {-+}}, where {ell} = e, {mu}, with data collected by the BABAR experiment at SLAC. This search demonstrates a novel technique in which we fully reconstruct the accompanying {bar B} in {Upsilon}(4S) {yields} B{bar 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 million B{bar B} pairs (342 fb{sup -1}), we find no evidence of signal in any of the decay modes. Branching fraction upper limits of {Beta}(B{sup +} {yields} e{sup +}{nu}) < 5.2 x 10{sup -6}, {Beta}(B{sup +} {yields} {mu}{sup +}{nu}) < 5.6 x 10{sup -6}, {Beta}(B{sup 0} {yields} e{sup +}{tau}{sup -}) < 2.8 x 10{sup -5} and {Beta}(B{sup 0} {yields} {mu}{sup +}{tau}{sup -}) < 2.2 x 10{sup -5}, are obtained at 90% confidence level.