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

Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the ${\mu }^{+}{\mu }^{-}\phantom{\rule{0.166667em}{0ex}}\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 \phantom{\rule{0.166667em}{0ex}}\gamma $ , $\psi \left(2S\right)\phantom{\rule{0.166667em}{0ex}}\gamma $ , and $Υ\left(1S,2S,3S\right)\phantom{\rule{0.166667em}{0ex}}\gamma $ are found to be $2.0×{10}^{-4}$ , $10.5×{10}^{-4}$ , and $\left(2.5,4.2,3.4\right)×{10}^{-4}$ , 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 $\left(1.1,1.3,2.4\right)×{10}^{-6}$ . An observed 95% CL interval of $\left(-133,175\right)$ is obtained for the ${\kappa }_{c}/{\kappa }_{\gamma }$ ratio of Higgs boson coupling modifiers, and a 95% CL interval of $\left(-37,40\right)$ is obtained for ${\kappa }_{b}/{\kappa }_{\gamma }$ .