Measurement of charmless semileptonic decays of B mesons

Using the CLEO II detector and a sample of 955 000 [Upsilon](4S) decays we have confirmed charmless semileptonic decays of [ital B] mesons. In the momentum interval 2.3--2.6 GeV/[ital c] we observe an excess of 107[plus minus]15[plus minus]11 leptons, which we attribute to [ital b][r arrow][ital ul][nu]. This result yields a model-dependent range of values for [vert bar][ital V][sub [ital u][ital b]]/[ital V][sub [ital c][ital b]][vert bar] that is lower than has been obtained in previous studies. For the inclusive spectator model of Altarelli [ital et] [ital al]. we find [vert bar][ital V][sub [ital u][ital b]]/[ital V][sub [ital c][ital b]][vert bar]=0.076[plus minus]0.008. Models that describe [ital b][r arrow][ital ul][nu] with a limited set of exclusive final states give [vert bar][ital V][sub [ital u][ital b]]/[ital V][sub [ital c][ital b]][vert bar]=0.06[minus]0.10.