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/ψγ , ψ(2S)γ , and Υ(1S,2S,3S)γ are found to be 2.0 × 10^-4 , 10.5 × 10^-4 , and (2.5 , 4.2 , 3.4) × 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 × 10^-6 , 2.4 × 10^-6 , and (1.1 , 1.3 , 2.4) × 10^-6 . An observed 95% CL interval of (-133, 175) is obtained for the κ_c/ κ_γ ratio of Higgs boson coupling modifiers, and a 95% CL interval of (-37, 40) is obtained for κ_b/ κ_γ .