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

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 $35.9{\mathrm{fb}}^{-1}$. 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 $Z\to J/\psi {\ell }^{+}{\ell }^{-}$ to the decay $Z\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ within a fiducial phase space is measured to be $\mathcal{B}(Z\to J/\psi {\ell }^{+}{\ell }^{-})/\mathcal{B}(Z\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-})=0.67\pm 0.18(\text{stat})\pm 0.05(\text{syst})$.

Figure 1

Leading-order Feynman diagram for the production of the $Z$ boson and its decay in the $Z\to J/\psi {\ell }^{+}{\ell }^{-}$ channel.

Figure 2

Distribution of invariant masses $m_{{\mu }^{+}{\mu }^{-}}$ vs $m_{{\mu }^{+}{\mu }^{-}{\ell }^{+}{\ell }^{-}}$ for the selected candidates. The values in the legend give the numbers of candidates per bin, which are also indicated by the sizes of the open black boxes.

Figure 3

Invariant mass distributions for the $\psi$ muon pairs (left) and for $\psi {\ell }^{+}{\ell }^{-}$ (right), for $Z\to \psi {\mu }^{+}{\mu }^{-}$ (upper) and $Z\to \psi e^{+}{e}^{-}$ (lower) candidates. In each histogram the data are represented by the points, with the vertical bars showing the statistical uncertainties, and the solid curve is the overall fit to the data. The shaded region corresponds to the signal yield, while the long-dashed lines are the $\psi$ meson signal from the $Z$ boson background (left) and the $Z$ boson signal from the $\psi$ meson background (right). The short-dashed line represents the combinatorial background.