Semileptonic Branching Fractions of Charged and Neutral B Mesons.

An examination of leptons in [Upsilon](4[ital S]) events tagged by reconstructed [ital B] meson decays yields semileptonic branching fractions of [ital b][sub [minus]]=(10.1[plus minus]1.8[plus minus]1.5)% for charged and [ital b][sub 0]=(10.9[plus minus]0.7[plus minus]1.1)% for neutral [ital B] mesons. This is the first measurement for charged [ital B] mesons. Assuming equality of the charged and neutral semileptonic widths, the ratio [ital b][sub [minus]]/[ital b][sub 0]=0.93[plus minus]0.18[plus minus]0.12 is equivalent to the ratio of lifetimes.