Observation of the Z → ψℓ^+ℓ^− Decay in pp Collisions at √s = 13  TeV

Abstract

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).