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

Using pp collision data corresponding to an integrated luminosity of about $140~\mathrm{fb}^{-1}$ collected by the CMS experiment at $\sqrt{s}= 13~\mathrm{TeV}$ in 2016--2018, the $\Lambda^{0}_{\mathrm{b}} \to \mathrm{J}/{\psi}\,\Xi^{-}\,\mathrm{K}^{+}$ decay is observed for the first time with a significance exceeding 5 standard deviations. The relative branching fraction, with respect to the $\Lambda^{0}_{\mathrm{b}} \to \psi(\mathrm{2S})\,\Lambda$ decay, is measured to be $\mathcal{B}(\Lambda^{0}_{\mathrm{b}} \to \mathrm{J}/{\psi}\,\Xi^{-}\,\mathrm{K}^{+})/\mathcal{B}(\Lambda^{0}_{\mathrm{b}} \to \psi(\mathrm{2S})\,\Lambda) = [2.5\pm 0.8\pm 0.9]\%$, where the first uncertainty is statistical and the second one is systematic.