Evidence for B ^-→Ds+K ^-ℓ ^-ν _̄ℓ and search for B ^-→Ds*+K ^-ℓ ^-ν _̄ℓ

We report measurements of the decays B ^-→Ds(*)+K ^-ℓ ^-ν _̄ℓ in a data sample containing 657×106BB̄ pairs collected with the Belle detector at the KEKB asymmetric-energy e ⁺e ^- collider. We observe a signal with a significance of 6σ for the combined D _s and Ds* modes and find the first evidence of the B ^-→Ds+K ^-ℓ ^-ν _̄ℓ decay with a significance of 3.4σ. We measure the following branching fractions: B(B ^-→Ds+K ^-ℓ ^-ν _̄ℓ)=(0.30±0.09(stat)-0.08+0.11(syst)) ×10 ^-3 and B(B ^-→Ds(*)+K ^-ℓ ^-ν _̄ℓ)=(0.59±0.12(stat) ±0.15(syst))×10 ^-3 and set an upper limit B(B ^-→Ds*+K ^-ℓ ^-ν _̄ℓ)<0.56×10 ^-3 at the 90% confidence level. We also present the first measurement of the Ds+K ^- invariant mass distribution in these decays, which is dominated by a prominent peak around 2.6GeV/c2.