Observation of the ${\Lambda }_{\text{b}}^{0}\to \text{J}/\psi {\Xi }^{-}{\text{K}}^{+}$ decay

Using proton–proton collision data corresponding to an integrated luminosity of $140\phantom{\rule{0.333333em}{0ex}}{\text{fb}}^{-1}$ collected by the CMS experiment at $\sqrt{s}=13\phantom{\rule{0.166667em}{0ex}}\text{Te}\phantom{\rule{-0.80002pt}{0ex}}\text{V}$ , the ${\Lambda }_{\text{b}}^{0}\to \text{J}/\psi {\Xi }^{-}{\text{K}}^{+}$ decay is observed for the first time, with a statistical significance exceeding 5 standard deviations. The relative branching fraction, with respect to the ${\Lambda }_{\text{b}}^{0}\to \psi \left(2\text{S}\right)\Lambda $ decay, is measured to be $B\left({\Lambda }_{\text{b}}^{0}\to \text{J}/\psi {\Xi }^{-}{\text{K}}^{+}\right)/B\left({\Lambda }_{\text{b}}^{0}\to \psi \left(2\text{S}\right)\Lambda \right)=\left[3.38±1.02±0.61±0.03\right]%$ , where the first uncertainty is statistical, the second is systematic, and the third is related to the uncertainties in $B\left(\psi \left(2\text{S}\right)\to \text{J}/\psi {\pi }^{+}{\pi }^{-}\right)$ and $B\left({\Xi }^{-}\to \Lambda {\pi }^{-}\right)$ .