Using the CLEO II detector and a sample of 955 000 Υ(4S) decays we have confirmed charmless semileptonic decays of B mesons. In the momentum interval 2.3–2.6 GeV/c we observe an excess of 107±15±11 leptons, which we attribute to b→ulν. This result yields a model-dependent range of values for ‖${\mathit{V}}_{\mathit{u}\mathit{b}}$/${\mathit{V}}_{\mathit{c}\mathit{b}}$‖ that is lower than has been obtained in previous studies. For the inclusive spectator model of Altarelli et al. we find ‖${\mathit{V}}_{\mathit{u}\mathit{b}}$/${\mathit{V}}_{\mathit{c}\mathit{b}}$‖=0.076±0.008. Models that describe b→ulν with a limited set of exclusive final states give ‖${\mathit{V}}_{\mathit{u}\mathit{b}}$/${\mathit{V}}_{\mathit{c}\mathit{b}}$‖=0.06-0.10.

• Received 7 September 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.4111