We report the first direct measurement of the inclusive branching fraction $\mathcal{B}(B_s\to D_{s}X)$ via $B_s$ tagging in $e^{+}{e}^{-}\to \mathrm{\Upsilon }(5\mathrm{S})$ events. Tagging is accomplished through a partial reconstruction of semileptonic decays $B_s\to D_{s}X\ell \nu$, where $X$ denotes unreconstructed additional hadrons or photons and $\ell$ is an electron or muon. With $121.4{\mathrm{fb}}^{-1}$ of data collected at the $\mathrm{\Upsilon }(5\mathrm{S})$ resonance by the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider, we obtain $\mathcal{B}(B_s\to D_{s}X)=(60.2\pm 5.8\pm 2.3)\%$, where the first uncertainty is statistical and the second is systematic.

• Received 21 June 2021
• Revised 3 December 2021
• Accepted 17 December 2021

DOI:https://doi.org/10.1103/PhysRevD.105.012004

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Published by the American Physical Society