Measurements of the Inclusive Semileptonic Branching Fraction of B Mesons at the UPSILON(4S) Resonance.

We present precision measurements of the inclusive semileptonic branching fraction of the B mesons from 2.057 fb^{-1} of Upsilon (4S) data collected with the CLEO-II detector. By fitting the inclusive lepton spectra to the refined quark model of Altarelli et al., we obtain {cal B}(B to Xlnu) = (10.56 +/-0.04+/-0.22)%. With a modified version of the form-factor model of Isgur et al., in which the fraction of Bto D**l nu is allowed to float, we find { cal B}(Bto Xlnu) = (10.96+/-0.07+/- 0.22)%. The original Isgur model yields a lower branching ratio, {cal B}(B to Xl nu) = (10.26+/-0.03 +/-0.22)%, but with a higher chi ^2. To reduce the dependence on theoretical models and sensitivity to possible non-BB decays of the Upsilon(4S), we have made a second measurement with dilepton events. In events with a high momentum lepton tag and an electron we use charge and kinematic correlations to separate the electron spectra of B decays and secondary charm decays. With a small extrapolation to account for the undetected part of the spectrum at low momentum, we obtain {cal B}(B to Xl nu) = (10.46 +/- 0.17 +/- 0.43)%. This measurement is largely independent of theoretical models and assumptions about possible non-BB decays of the Upsilon(4S). Based on our branching ratio results, we have also measured the CKM matrix element | V_{cb}| with precision and confirmed the CLEO measurement of | V_{ub}|.