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 final state with a proton–proton collision data sample corresponding to an integrated luminosity of 139 fb^{-1} collected at 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/psi ,gamma , psi (2S),gamma , and Upsilon (1S,2S,3S),gamma are found to be 2.0times 10^{-4}, 10.5times 10^{-4}, and (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 1.2times 10^{-6}, 2.4times 10^{-6}, and (1.1,1.3,2.4)times 10^{-6}. An observed 95% CL interval of (-133,175) is obtained for the kappa _c/kappa _gamma ratio of Higgs boson coupling modifiers, and a 95% CL interval of (-37,40) is obtained for kappa _b/kappa _gamma .