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

We present a search for the decay B + → l + ν l (l = T , μ, ore) in (458.9 ± 5.1) × 10 6 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 - → D 0 l - νX. Using the method of Feldman and Cousins, we obtain B(B + → τ + ν τ = (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 f 2 B = (62 ± 31) × 10 3 MeV 2 . We find no evidence for B + → e + ν e and B + → μ + ν μ and set upper limits at the 90% C.L. B(B + → e + ν e ) < 0.8 X 10 -5 and B(B + → μ + ν + μ) < 1.1 × 10- 5 .