Reconstruction of semileptonic b ---> u decays

Abstract Using the ARGUS detector at the e + e − storage ring DORIS II at DESY we have updated our result on charmless semileptonic B decays and searched for direct evidence for b→u transitions through reconstruction of complete events. This approach has successfully identified two ϒ (4 S ) events which contain B meson decays to a non-charm final state. In one case, a B 0 meson is found in the channel π + μ − ν , while in the other a decay B + → ωμ + ν is seen. These events show that the previously observed signal of charmless semileptonic B decays is due to b→u transitions from which we obtain | V ub V cb | = 0.11±0.012 using the model of Altarelli et al.