Observation ofB-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{→}cl{ν} is negligible, the average of the measured b{→}ue{ν} and b{→}u{μ}{ν} partial branching ratios is {Δ}Bub(2.4,2.6) =(1.8±{}0.4±{}0.3)×{}10^-4. Inclusion of data from the interval 2.2--2.4 GeV/c, where the lepton yield is dominated by b{→}cl{ν}, gives {Δ}Bub(2.2,2.6) =(3.3±{}0.8±{}0.8)×{}10^-4. {}Vub/Vcb{} depends on the theoretical model of b{→}ul{ν} decay and is approximately 0.1.