Abstract: This Letter presents the observation of the rare Z boson decay Z?? + -. Here, ? represents contributions from direct J/? and ?(2S)?J/?X, + - is a pair of electrons or muons, and the J/? meson is detected via its decay to ?+?-. 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 ?(2S)?J/?X contribution, the ratio of the branching fraction of the exclusive decay Z?J/? + - to the decay Z??+?-?+?- within a fiducial phase space is measured to be B(Z?J/? + -)/B(Z??+?-?+?-)=0.67±0.18(stat)±0.05(syst).

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