Observation of the Λb0→J/ψΞ-K+ decay

Using proton–proton collision data corresponding to an integrated luminosity of 140fb^-1 collected by the CMS experiment at s=13TeV, the Λ_b⁰→J/ψΞ^-K⁺ decay is observed for the first time, with a statistical significance exceeding 5 standard deviations. The relative branching fraction, with respect to the Λ_b⁰→ψ(2S)Λ decay, is measured to be B(Λ_b⁰→J/ψΞ^-K⁺)/B(Λ_b⁰→ψ(2S)Λ)=[3.38±1.02±0.61±0.03]%, where the first uncertainty is statistical, the second is systematic, and the third is related to the uncertainties in B(ψ(2S)→J/ψπ⁺π^-) and B(Ξ^-→Λπ^-).