Observation of the Z → ψ ℓ + ℓ - decay in pp collisions at v 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 ( 2 S ) → J / ψ X , 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 ( 2 S ) → J / ψ X contribution, the ratio of the branching fraction of the exclusive decay Z→ J / ψ ℓ + ℓ − to the decay Z→ μ + μ − μ + μ − within a fiducial phase space is measured to be B(Z→ J / ψ ℓ + ℓ − ) / B ( Z → μ + μ − μ + μ − ) = 0.67 ± 0.18 ( stat ) ± 0.05 ( syst ). ©2018