Observation and branching fraction measurement of the decay Ξ −b → Λ0bπ

The decay Ξ−𝑏→Λ0𝑏⁢𝜋− is observed using a proton-proton collision data sample collected at center-of-mass energy √𝑠=13  TeV with the LHCb detector, corresponding to an integrated luminosity of 5.5  fb−1. This process is mediated by the 𝑠→𝑢⁢¯𝑢⁢𝑑 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→Λ+𝑐⁢𝜋− and Λ0𝑏→Λ+𝑐⁢𝜋−⁢𝜋+⁢𝜋−, the relative production ratio is measured to be (𝑓Ξ−𝑏/𝑓Λ0𝑏)⁢ℬ⁡(Ξ−𝑏→Λ0𝑏⁢𝜋−)=(7.3±0.8±0.6)×10−4. Here the uncertainties are statistical and systematic, respectively, and 𝑓Ξ−𝑏⁡(𝑓Λ0𝑏) is the fragmentation fraction for a 𝑏 quark into a Ξ−𝑏 (Λ0𝑏) baryon. Using an independent measurement of 𝑓Ξ−𝑏/𝑓Λ0𝑏, the branching fraction ℬ⁡(Ξ−𝑏→Λ0𝑏⁢𝜋−)=(0.89±0.10±0.07±0.29)% is obtained, where the last uncertainty is due to the assumed SU(3) flavor symmetry in the determination of 𝑓Ξ−
𝑏/𝑓Λ0𝑏.