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

The decay Ξ−b→Λ0bπ− 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→uu¯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+cπ− and Λ0b→Λ+cπ−π+π−, the relative production ratio is measured to be (fΞ−b/fΛ0b)B(Ξ−b→Λ0bπ−)=(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→Λ0bπ−)=(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.