Search for B+→ℓ+νℓ recoiling against B−→D0ℓ−¯νX

We present a search for the decay B + → ℓ + ν ℓ ( ℓ = τ , μ , or e ) in ( 458.9 ± 5.1 ) × 10 6 B ¯¯¯ B 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 − ¯ ν X . 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 2.3 σ . 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 × 10 − 5 and B ( B + → μ + ν μ ) < 1.1 × 10 − 5