Abstract
We present a search for the decay B+ → ℓ+ν ( = τ, μ, or e) in (458.9±5.1)×106 Υ(4S) decays recorded with the BABAR detector at the SLAC PEP-II B-Factory. A sample of events with one reconstructed exclusive semi-leptonic B decay (B- → D0ℓ -$\bar{v}$X) is selected, and in the recoil a search for B+ →ℓ +νℓ signal is performed. The τ is identified in the following channels: τ+ → e+νe$\bar{v}$τ , τ+ → μ+νμ$\bar{v}$τ , τ+ → π+$\bar{v}$τ , and τ+ → π+π0$\bar{v}$τ . The analysis strategy and the statistical procedure is set up for branching fraction extraction or upper limit determination. We determine from the dataset a preliminary measurement of B(B+ → τ+ντ) = (1.8 ± 0.8 ± 0.1) × 10-4, which excludes zero at 2.4σ, and fB = 255 ± 58 MeV. Combination with the hadronically tagged measurement yields B(B+ → τ+ντ) = (1.8 ± 0.6) × 10-4. We also set preliminary limits on the branching fractions at B(B+ → e+νe) < 7.7 × 10-6 (90% C.L.), B(B+ → μ+νμ) < 11 × 10-6 (90% C.L.), and B(B+ → τ+ντ ) < 3.2 × 10-4(90% C.L.).
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