Measurement of the B semileptonic branching fraction with lepton tags

Abstract

We have used the CLEO II detector and 2.06 fb(-1) of Y(4S) data to measure the B-meson semileptonic branching fraction. The B --> Xe nu momentum spectrum was obtained over nearly the full momentum range by using charge and kinematic correlations in events with a high-momentum lepton tag and an additional electron. We find B(B --> Xe nu) = (10.49 +/- 0.17 +/- 0.43)%, with overall systematic uncertainties less than those of untagged single-lepton measurements. We use this result to calculate the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element V-cb and to set an upper limit on the fraction of Y(4S) decays to final states other than B (B) over bar.