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 κc/κγ ratio of Higgs boson coupling modifiers, and a 95% CL interval of (−37,40) is obtained for κb/κγ.