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

Abstract

This Letter presents the observation of the rare Z boson decay Z $\to \psi {\ell }^{+}{\ell }^{-}$. Here, $\psi$ represents contributions from direct J/$\psi$ and $\psi$(2S) $\to$ J/$\psi X$, ${\ell }^{+}{\ell }^{-}$ 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 $\to \psi {\ell }^{+}{\ell }^{-}$ 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).

Replaced with the published version. Added the journal reference and the DOI. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/BPH-16-001 (CMS Public Pages)