Observation and branching fraction measurement of the decay Ξ_b⁻ → Λ⁰_bπ⁻

The decay Ξ⁻_b → Λ⁰_bπ⁻ 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 Λ⁰_b decay modes, Λ⁰_b → Λ^þ_c π⁻ and Λ⁰_b → Λ^þ_c π⁻π^þπ⁻, the relative production ratio is measured to be ðf_Ξ−_b =f_Λ0 _bÞBðΞ⁻_b → Λ⁰_bπ⁻Þ ¼ ð7.3 ± 0.8 ± 0.6Þ × 10⁻⁴. Here the uncertainties are statistical and systematic, respectively, and f_Ξ−_b ðf_Λ0 _bÞ is the fragmentation fraction for a b quark into a Ξ⁻_b (Λ⁰_b) baryon. Using an independent measurement of f_Ξ− _b =f_Λ0 _b, the branching fraction BðΞ⁻_b → Λ⁰_bπ⁻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 f_Ξ− _b =f_Λ0 _b,.