UC Irvine

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