Observation of<i>B</i>-meson semileptonic decays to noncharmed final states

We report the first evidence of charmless semileptonic decays of B mesons. In the momentum interval 2.4--2.6 GeV/c where the background from b\ensuremath{\rightarrow}cl\ensuremath{\nu} is negligible, the average of the measured b\ensuremath{\rightarrow}ue\ensuremath{\nu} and b\ensuremath{\rightarrow}u\ensuremath{\mu}\ensuremath{\nu} partial branching ratios is \ensuremath{\Delta}${B}_{\mathrm{ub}}$(2.4,2.6) =(1.8\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. Inclusion of data from the interval 2.2--2.4 GeV/c, where the lepton yield is dominated by b\ensuremath{\rightarrow}cl\ensuremath{\nu}, gives \ensuremath{\Delta}${B}_{\mathrm{ub}}$(2.2,2.6) =(3.3\ifmmode\pm\else\textpm\fi{}0.8\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. \ensuremath{\Vert}${V}_{\mathrm{ub}}$/${V}_{\mathrm{cb}}$\ensuremath{\Vert} depends on the theoretical model of b\ensuremath{\rightarrow}ul\ensuremath{\nu} decay and is approximately 0.1.