Аннотация:Using proton–proton collision data corresponding to an integrated luminosity of 140 fb−1 collected by the CMS experiment at √s = 13 TeV, the Λ^0_b → 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^0_b → ψ(2S)Λ decay, is measured to be B(Λ^0_b → J/ψΞ−K+)/B(Λ^0_b → 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−).