Searches for exclusive Higgs and Boson Decays Into / , (2 ) , and ϒ( ) At with the ATLAS detector

Searches for the exclusive decays of the Higgs and Z bosons into a $J/\psi $, $\psi \left(2S\right)$, or $\Upsilon \left(nS\right)$ ($n=1,2,3$) meson and a photon are performed with a pp collision data sample corresponding to an integrated luminosity of $36.1\phantom{\rule{0.25em}{0ex}}{\text{fb}}^{-1}$ collected at $\sqrt{s}=13\phantom{\rule{0.25em}{0ex}}\text{TeV}$ with the ATLAS detector at the CERN Large Hadron Collider. No significant excess of events is observed above the expected backgrounds, and 95% confidence-level upper limits on the branching fractions of the Higgs boson decays to $J/\psi \phantom{\rule{0.2em}{0ex}}\gamma $, $\psi \left(2S\right)\phantom{\rule{0.2em}{0ex}}\gamma $, and $\Upsilon \left(nS\right)\phantom{\rule{0.2em}{0ex}}\gamma $ of $3.5×{10}^{-4}$, $2.0×{10}^{-3}$, and $\left(4.9,5.9,5.7\right)×{10}^{-4}$, respectively, are obtained assuming Standard Model production. The corresponding 95% confidence-level upper limits for the branching fractions of the Z boson decays are $2.3×{10}^{-6}$, $4.5×{10}^{-6}$ and $\left(2.8,1.7,4.8\right)×{10}^{-6}$, respectively.