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

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