Limit on the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>b</mml:mi><mml:mo>→</mml:mo><mml:mi>u</mml:mi></mml:math>Coupling from Semileptonic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>B</mml:mi></mml:math>Decay

We have used the momentum spectrum of leptons produced in semileptonic $B$-meson decays to set a 90%-confidence-level upper limit on $\frac{\ensuremath{\Gamma}(b\ensuremath{\rightarrow}\mathrm{ul}\ensuremath{\nu})}{\ensuremath{\Gamma}(b\ensuremath{\rightarrow}\mathrm{cl}\ensuremath{\nu})}$ of 4%. We also measure the semileptonic branching fractions of the $B$ meson to be (12.0\ifmmode\pm\else\textpm\fi{}0.7\ifmmode\pm\else\textpm\fi{}0.5)% for electrons and (10.8\ifmmode\pm\else\textpm\fi{}0.6\ifmmode\pm\else\textpm\fi{}1.0)% for muons.