Observation of the Λ⁰_b → 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 Λ⁰_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 Λ⁰_b → ψ(2S)Λ decay, is measured to be B(Λ⁰_b → J/ψΞ⁻K⁺) / B(Λ⁰_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⁻).