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