Searches for the decays B0→l±τ∓ and B+→l+ν (l=e, μ) using hadronic tag reconstruction

Abstract

We present searches for the leptonic decays B + → ℓ + ν and the lepton flavor violating decays B 0 → ℓ ± τ ∓ , where ℓ = e , μ , with data collected by the BABAR experiment at SLAC. This search demonstrates a novel technique in which we fully reconstruct the accompanying ¯¯¯ B in Υ ( 4 S ) → B ¯¯¯ 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 × 10 6 B ¯¯¯ B pairs ( 342 fb − 1 ), we find no evidence of signal in any of the decay modes. Branching fraction upper limits of B ( B + → e + ν ) < 5.2 × 10 − 6 , B ( B + → μ + ν ) < 5.6 × 10 − 6 , B ( B 0 → e + τ − ) < 2.8 × 10 − 5 and B ( B 0 → μ + τ − ) < 2.2 × 10 − 5 , are obtained at 90% confidence level.