Measurement of B(B-s -> DsX) with B-s semileptonic tagging

We report the first direct measurement of the inclusive branching fraction B(B-s -> DsX) via B-s tagging in e(+)e(-) -> gamma(5S) events. Tagging is accomplished through a partial reconstruction of semileptonic decays B-s -> D(s)Xl nu, where X denotes unreconstructed additional hadrons or photons and l is an electron or muon. With 121.4 fb(-1) of data collected at the gamma(5S) resonance by the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider, we obtain B(B-s -> DsX) = (60.2 +/- 5.8 +/- 2.3)%, where the first uncertainty is statistical and the second is systematic.