Searches for the decays $B^0 \to \ell^\pm \tau^\mp$ and $B^ + \to \ell^ + \nu$ (l e, $\mu^ ) $ using hadronic tag reconstruction

We present searches for the leptonic decays B+→ℓ+ν and the lepton flavor violating decays B0→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(4S)→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(B0→e+τ−)<2.8×10−5 and B(B0→μ+τ−)<2.2×10−5, are obtained at 90% confidence level.