The observation is presented of the Z boson rare decay to a ψ meson and two oppositely charged same-flavor leptons, l+l−, where ψ represents the sum of J/ψ and ψ(2S) → J/ψ X, and l = μ, e, is presented. The data sample of proton-proton collisions at a center-of-mass energy of 13 TeV corresponds to an integrated luminosity of 35.9fb−1 accumulated by the CMS experiment at the CERN LHC. The signal is observed with a significance in excess of 5 standard deviations. Removing contributions from ψ(2S) decays to J/ψ, the signal is interpreted as being entirely from Z → J/ψ l+l−, with its fiducial branching fraction relative to that of the decay Z → μ+μ−μ+μ− measured to be
B(Z → J/ψ l+l−)/B(Z → μ+μ−μ+μ−) = 0.70 ± 0.18 (stat) ± 0.05 (syst)
This result is obtained with the assumption of no J/ψ polarization. Extreme polarization scenarios give a variation of the fiducial branching fraction measurement of (−22 to +24)%.