Measurement of charmless semileptonic decays of<i>B</i>mesons

Using the CLEO II detector and a sample of 955 000 \ensuremath{\Upsilon}(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\ifmmode\pm\else\textpm\fi{}15\ifmmode\pm\else\textpm\fi{}11 leptons, which we attribute to b\ensuremath{\rightarrow}ul\ensuremath{\nu}. This result yields a model-dependent range of values for \ensuremath{\Vert}${\mathit{V}}_{\mathit{u}\mathit{b}}$/${\mathit{V}}_{\mathit{c}\mathit{b}}$\ensuremath{\Vert} that is lower than has been obtained in previous studies. For the inclusive spectator model of Altarelli et al. we find \ensuremath{\Vert}${\mathit{V}}_{\mathit{u}\mathit{b}}$/${\mathit{V}}_{\mathit{c}\mathit{b}}$\ensuremath{\Vert}=0.076\ifmmode\pm\else\textpm\fi{}0.008. Models that describe b\ensuremath{\rightarrow}ul\ensuremath{\nu} with a limited set of exclusive final states give \ensuremath{\Vert}${\mathit{V}}_{\mathit{u}\mathit{b}}$/${\mathit{V}}_{\mathit{c}\mathit{b}}$\ensuremath{\Vert}=0.06-0.10.