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

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