Searches for the decays B^{0}→l^{±}τ^{∓} and B^{+}→l^{+}ν (l=e, μ) using hadronic tag reconstruction

We present searches for the leptonic decays B+-> l(+)nu and the lepton flavor violating decays B-0 -> l(+/-)tau(-/+), where l = e, mu, with data collected by the BABAR experiment at SLAC. This search demonstrates a novel technique in which we fully reconstruct the accompanying (B) over bar in Gamma(4S)-> B (B) over bar 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 378x10(6) B (B) over bar pairs (342 fb(-1)), we find no evidence of signal in any of the decay modes. Branching fraction upper limits of B(B+ -> e(+)nu) <5.2 x 10(-6), B(B+ -> mu(+)nu) <5.6 x 10(-6), B(B-0 -> e(+)tau(-)) <2.8 x 10(-5) and B(B-0 -> mu(+)tau(-)) <2.2 x 10(-5), are obtained at 90% confidence level.