Observation of B Meson Semileptonic Decays to Noncharmed Final States

We report the first evidence of charmless semileptonic decays of {ital B} mesons. In the momentum interval 2.4--2.6 GeV/{ital c} where the background from {ital b}{r arrow}{ital cl}{nu} is negligible, the average of the measured {ital b}{r arrow}{ital ue}{nu} and {ital b}{r arrow}{ital u}{mu}{nu} partial branching ratios is {Delta}{ital B}{sub {ital ub}}(2.4,2.6) =(1.8{plus minus}0.4{plus minus}0.3){times}10{sup {minus}4}. Inclusion of data from the interval 2.2--2.4 GeV/{ital c}, where the lepton yield is dominated by {ital b}{r arrow}{ital cl}{nu}, gives {Delta}{ital B}{sub {ital ub}}(2.2,2.6) =(3.3{plus minus}0.8{plus minus}0.8){times}10{sup {minus}4}. {vert bar}{ital V}{sub {ital ub}}/V{sub cb}{vert bar} depends on the theoretical model of {ital b}{r arrow}{ital ul}{nu} decay and is approximately 0.1.