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 $Z$→ $ψ {ℓ^+ℓ^-}$. 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 $B(Z→J/ψℓ^+ℓ^−) /B(Z→μ^+μ^−μ^+μ^−)=0.67±0.18(stat)±0.05(syst)$.