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

Abstract Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the $$\mu ^+\mu ^-\,\gamma $$ μ + μ - γ final state with a proton–proton collision data sample corresponding to an integrated luminosity of 139 fb $$^{-1}$$ - 1 collected at $$\sqrt{s}=13$$ 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 $$J/\psi \,\gamma $$ J / ψ γ , $$\psi (2S)\,\gamma $$ ψ ( 2 S ) γ , and $$\Upsilon (1S,2S,3S)\,\gamma $$ Υ ( 1 S , 2 S , 3 S ) γ are found to be $$2.0\times 10^{-4}$$ 2.0 × 10 - 4 , $$10.5\times 10^{-4}$$ 10.5 × 10 - 4 , and $$(2.5,4.2,3.4)\times 10^{-4}$$ ( 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 $$1.2\times 10^{-6}$$ 1.2 × 10 - 6 , $$2.4\times 10^{-6}$$ 2.4 × 10 - 6 , and $$(1.1,1.3,2.4)\times 10^{-6}$$ ( 1.1 , 1.3 , 2.4 ) × 10 - 6 . An observed 95% CL interval of $$(-133,175)$$ ( - 133 , 175 ) is obtained for the $$\kappa _c/\kappa _\gamma $$ κ c / κ γ ratio of Higgs boson coupling modifiers, and a 95% CL interval of $$(-37,40)$$ ( - 37

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