Observation of semileptonic charmless bottom meson decays with the ALEPH detector at LEP

Charmless semileptonic decays of bottom mesons were searched for in approximately 4 million hadronic decays of the Z 0 collected by the Aleph detector during the period 1991-95. Separation of these decays from the dominant charmed decay was achieved by requiring the energy of the lepton to be above the kinematical end point of the charmed decay process. In the energy region above 32.5 G e V, 42 : 6 17 : 8 9 : 5 electron events were observed in excess of carefully supressed and assessed backgrounds and are attributed to b -> u transitions.