Search for B+→l+νl recoiling against B-→D0l-νX

We present a search for the decay B+→l+νl(l=τ,μ, or e) in (458.9±5.1)x106 BB 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-νX. Using the method of Feldman and Cousins, we obtain B(B+→τ+ντ)=(1.7±0.8±0.2)x10-4, 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)x103 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)<0.8x10-5 and B(B+→μ+νμ)<1.1x10-5.