Observation of the $\Lambda_{\mathrm{b}} \to \mathrm{J}/\psi \Lambda \phi$ decay in proton-proton collisions at $\sqrt{s}=$ 13 TeV

We report a first observation of the $\Lambda_{\mathrm{b}} \to \mathrm{J}/\psi \Lambda \phi$ decay using proton-proton collision data collected at $\sqrt{s}$= 13 TeV by the CMS experiment at the LHC, and corresponding to an integrated luminosity of 60 $\mathrm{fb}^{-1}$. The ratio of the branching fractions ${\cal B}(\Lambda_{\mathrm{b}} \to \mathrm{J}/\psi \Lambda \phi)/{\cal B}(\Lambda_{\mathrm{b}}\to \psi\mathrm{(2S)} \Lambda)$ is measured to be $(8.26\pm0.90~\mathrm{(stat)} \pm0.68~\mathrm{(syst)}\pm0.11({\cal B}))\times 10^{-2}$, where the first uncertainty is statistical, the second is systematic, and the third is due to the uncertainties in the world-average branching fractions.