Observation of the decay B- D(s)((*)+) K- ℓ- nu(ℓ)

We report the observation of the decay B- D(s)((*)+) K- ℓ- nu(ℓ) based on 342fb(-1) of data collected at the Upsilon(4S) resonance with the BABAR detector at the PEP-II e+ e- storage rings at SLAC. A simultaneous fit to three D(s)(+) 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- D(s)((*)+) K- ℓ- nu(ℓ) with a significance greater than 5 standard deviations (including systematic uncertainties) and measure its branching fraction to be B(B- D(s)((*)+) K- ℓ- nu(ℓ)) = [6.13(-1.03)(+1.04)(stat)0.43(syst)0.51(B(D(s)))]*10(-4), where the last error reflects the limited knowledge of the D(s) branching fractions.