Observation of the Z $\to \psi {\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ decay in pp collisions at $\sqrt{s}=$ 13 TeV

This Letter presents the observation of the rare Z boson decay Z $\to \psi {\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$. Here, $\psi$ represents contributions from direct J/$\psi$ and $\psi$(2S) $\to$ J/$\psi X$, ${\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ is a pair of electrons or muons, and the J/$\psi$ meson is detected via its decay to ${\mu }^{+}{\mu }^{-}$. The sample of proton-proton collision data, collected by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, corresponds to an integrated luminosity of 35.9 fb${}^{-1}$. The signal is observed with a significance in excess of 5 standard deviations. After subtraction of the $\psi$(2S) $\to$ J/$\psi X$ contribution, the ratio of the branching fraction of the exclusive decay Z $\to \psi {\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ to the decay Z $\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ within a fiducial phase space is measured to be $\mathcal{B}($Z $\to\psi\ell^+\ell^-) / $\mathcal{B}$($Z$\to\mu^+\mu^-\mu^+\mu^-) =$0.67$\pm$0.18(stat)$\pm$ 0.05 (syst).