Search for B^{+}→ℓ^{+}ν_{ℓ} recoiling against B^{-}→D^{0}ℓ^{-}ν[over ¯]X

We present a search for the decay B+→ℓ+νℓ(ℓ=τ,μ, or e) in (458.9±5.1)×106 BB̅ pairs recorded with the BABAR detector at the PEP-II B-factory. We search for these B decays in a sample of B+B- events where one B-meson is reconstructed as B-→D0ℓ-ν̅ X. Using the method of Feldman and Cousins, we obtain B(B+→τ+ντ)=(1.7±0.8±0.2)×10-4, which excludes zero at 2.3σ. We interpret the central value in the context of the standard model and find the B meson decay constant to be fB2=(62±31)×103  MeV2. We find no evidence for B+→e+νe and B+→μ+νμ and set upper limits at the 90% C.L. B(B+→e+νe)<0.8×10-5 and B(B+→μ+νμ)<1.1×10-5.