The decay ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-}$ is observed using a proton-proton collision data sample collected at center-of-mass energy $\sqrt{s}=13\mathrm{TeV}$ with the LHCb detector, corresponding to an integrated luminosity of $5.5{\mathrm{fb}}^{-1}$. This process is mediated by the $s\to u\overline{u}d$ quark-level transition, where the $b$ quark in the ${\mathrm{\Xi }}_b^{-}$ baryon is a spectator in the decay. Averaging the results obtained using the two ${\mathrm{\Lambda }}_b^0$ decay modes, ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}$ and ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$, the relative production ratio is measured to be $(f_{{\mathrm{\Xi }}_b^{-}}/f_{{\mathrm{\Lambda }}_b^0})\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})=(7.3\pm 0.8\pm 0.6)\times {10}^{-4}$. Here the uncertainties are statistical and systematic, respectively, and $f_{{\mathrm{\Xi }}_b^{-}}(f_{{\mathrm{\Lambda }}_b^0})$ is the fragmentation fraction for a $b$ quark into a ${\mathrm{\Xi }}_b^{-}$ (${\mathrm{\Lambda }}_b^0$) baryon. Using an independent measurement of $f_{{\mathrm{\Xi }}_b^{-}}/f_{{\mathrm{\Lambda }}_b^0}$, the branching fraction $\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})=(0.89\pm 0.10\pm 0.07\pm 0.29)\%$ is obtained, where the last uncertainty is due to the assumed SU(3) flavor symmetry in the determination of $f_{{\mathrm{\Xi }}_b^{-}}/f_{{\mathrm{\Lambda }}_b^0}$.

• Received 18 July 2023
• Accepted 23 August 2023

DOI:https://doi.org/10.1103/PhysRevD.108.072002

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

© 2023 CERN, for the LHCb Collaboration