Observation of the Decay B- → Ds(*)+K-(ell)-(bar ν)#ell#

We report the observation of the decay B- → Ds(*)+ K-(ell)-(bar ν)#ell# based on 342 fb-1 of data collected at the Υ(4S) resonance with the BABAR detector at the PEP-II e+e- storage rings at SLAC. A simultaneous fit to three Ds+ decay chains is performed to extract the signal yield from measurements of the squared missing mass in the B meson decay. We observe the decay B- → Ds(*)+ K-(ell)-(bar ν)#ell# with a significance greater than five standard deviations (including systematic uncertainties) and measure its branching fraction to be Β(B- → Ds(*)+ K-(ell)-(bar ν)#ell#) = (6.13-1.03+1.04(stat.) ± 0.43(syst.) ± 0.51(Β(Ds))) x 10-4, where the last error reflects the limited knowledge of the Ds branching fractions.