Ju n 19 94 CLNS 94 / 1285 CLEO 94-27 Measurement of the B̄ → D ∗ l ν̄ Branching Fractions and

We study the exclusive semileptonic B meson decays B− → D∗0 l− ν̄ and B̄0 → D∗+ l− ν̄ using data collected with the CLEO II detector at the Cornell Electron-positron Storage Ring (CESR). We present measurements of the branching fractions, B(B̄0 → D∗+ l− ν̄) = (0.5/f00)[4.49 ± 0.32(stat.) ± 0.39(sys.)]% and B(B− → D∗0 l− ν̄) = (0.5/f+−)[5.13 ± 0.54(stat.) ± 0.64(sys.)]%, where f00 and f+− are the neutral and charged B meson production fractions at the Υ(4S) resonance, respectively. Assuming isospin invariance and taking the ratio of charged to neutral B meson lifetimes measured at higher energy machines, we determine the ratio f+−/f00 = 1.04 ± 0.13(stat.) ± 0.12(sys.) ± 0.10(lifetime); further assuming f+− + f00 = 1 we also determine the partial width Γ(B̄ → D∗ l ν̄) = ∗Permanent address: University of Hawaii at Manoa †Permanent address: INP, Novosibirsk, Russia