Searches for exclusive Higgs and Z boson decays into a vector quarkonium state and a photon using 139 fb^–1 of ATLAS √s = 13 TeV proton-proton collision data

Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the μ⁺μ^-γ final state with a proton–proton collision data sample corresponding to an integrated luminosity of 139 fb^-1 collected at √s = 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected backgrounds. The 95% confidence-level upper limits on the branching fractions of the Higgs boson decays into J/ψΥ,ψ(2S) γ, and ϒ (1S, 2S, 3S) γ are 2.0 x 10-⁴ found to be 10.5 x 10^-4, and (2.5, 4.2, 3.4) x 10^-4, respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the Z boson decays are 1.2 x 10^-6, 2.4 x 10^-6, and (1.1, 1.3, 2.4) x 10^-6. An observed 95% CL interval of (-133, 175) is obtained for the K_c/K_γ
ratio of Higgs boson coupling modifiers, and a 95% CL interval of 9-37, 40) is obtained for K_b/K_γ.