Observation and branching fraction measurement of the decay <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msubsup><mml:mi mathvariant="normal">Ξ</mml:mi><mml:mi>b</mml:mi><mml:mo>−</mml:mo></mml:msubsup><mml:mo stretchy="false">→</mml:mo><mml:msubsup><mml:mi mathvariant="normal">Λ</mml:mi><mml:mi>b</mml:mi><mml:mn>0</mml:mn></mml:msubsup><mml:msup><mml:mi>π</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:math>

The decay Ξ−b → Λ0bπ− is observed using a proton-proton collision data sample collected at center-of-ffiffi mass energy ps ¼ 13 TeV with the LHCb detector, corresponding to an integrated luminosity of 5.5 fb−1. This process is mediated by the s → uūd quark-level transition, where the b quark in the Ξ−b baryon is a spectator in the decay. Averaging the results obtained using the two Λ0b decay modes, Λ0b → Λþc π− and Λ0b → Λþc π−πþπ−, the relative production ratio is measured to be ðfΞ−b =fΛ0 bÞBðΞ−b → Λ0bπ−Þ ¼ ð7.3 ± 0.8 ± 0.6Þ × 10−4. Here the uncertainties are statistical and systematic, respectively, and fΞ−b ðfΛ0 bÞ is the fragmentation fraction for a b quark into a Ξ−b (Λ0b) baryon. Using an independent measurement of fΞ− b =fΛ0 b, the branching fraction BðΞ−b → Λ0bπ−Þ ¼ ð0.89 ± 0.10 ± 0.07 ± 0.29Þ% is obtained, where the last uncertainty is due to the assumed SU(3) flavor symmetry in the determination of fΞ− b =fΛ0 b,