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→clν is negligible, the average of the measured b→ueν and b→uμν partial branching ratios is Δ$B_{\mathrm{ub}}$(2.4,2.6) =(1.8±0.4±0.3)×${10}^{\mathrm{-}4}$. Inclusion of data from the interval 2.2–2.4 GeV/c, where the lepton yield is dominated by b→clν, gives Δ$B_{\mathrm{ub}}$(2.2,2.6) =(3.3±0.8±0.8)×${10}^{\mathrm{-}4}$. ‖$V_{\mathrm{ub}}$/$V_{\mathrm{cb}}$‖ depends on the theoretical model of b→ulν decay and is approximately 0.1.

• Received 8 November 1989

DOI:https://doi.org/10.1103/PhysRevLett.64.16