Search for B+→ℓ+νℓrecoiling against B-→ℓ-ν̄X

We present a search for the decay B+→ℓ +νℓ(ℓ=τ,μ,ore) in (458.9±5.1)×106 BB̄ 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 -ν̄X. 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)×103MeV2. We find no evidence for B +→e+νe and B+→μ+ν μ 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.