Measurement of <math><mrow><mi>B</mi><mo>(</mo><msub><mrow><mi>B</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>→</mo><msub><mrow><mi>D</mi></mrow><mrow><mi>s</mi></mrow></msub><mi>X</mi><mo>)</mo></mrow></math> with <math><msub><mi>B</mi><mi>s</mi></msub></math> semileptonic tagging

We report the first direct measurement of the inclusive branching fraction B(Bs→DsX) via Bs tagging in e+e−→ϒ(5S) events. Tagging is accomplished through a partial reconstruction of semileptonic decays Bs→DsXℓν, 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 ϒ(5S) resonance by the Belle detector at the KEKB asymmetric-energy e+e− collider, we obtain B(Bs→DsX)=(60.2&#177;5.8&#177;2.3)%, where the first uncertainty is statistical and the second is systematic.