Measurement of ${mathcal B}$($B_s rightarrow D_s X$) with $B_s$ Semileptonic Tagging

We report the first direct measurement of the inclusive branching fraction ${mathcal B}(B_s rightarrow D_s X)$ via $B_s$ tagging in $e^+e^-toUpsilon$(5S) events. Tagging is accomplished through a partial reconstruction of semileptonic decays $B_s rightarrow D_s X ell u$, where $X$ denotes unreconstructed additional hadrons or photons and $ell$ is an electron or muon. With 121.4 fb$^{-1}$ of data collected at the $Upsilon$(5S) resonance by the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider, we obtain ${mathcal B}(B_s rightarrow D_s X)$ = $(61.6 pm 5.3 pm 2.1)$%, where the first uncertainty is statistical and the second is systematic.