Searches for exclusive Higgs and $Z$ boson decays into $J\mathrm{/}\psi \gamma$, $\psi (2S)\gamma$, and $\mathrm{{\rm Y}}(nS)\gamma$ at $\sqrt{s}=13$ TeV with the ATLAS detector

Searches for the exclusive decays of the Higgs and $Z$ bosons into a $J\mathrm{/}\psi$, $\psi (2S)$, or $\mathrm{{\rm Y}}(nS)$ ($n=1,2,3$) meson and a photon are performed with a $pp$ collision data sample corresponding to an integrated luminosity of 36.1 fb${}^{-1}$ collected at $\sqrt{s}=13$ 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 $J\mathrm{/}\psi \gamma$, $\psi (2S)\gamma$, and $\mathrm{{\rm Y}}(nS)\gamma$ of $3.5\times {10}^{-4}$, $2.0\times {10}^{-3}$, and $(4.9,5.9,5.7)\times {10}^{-4}$, respectively, are obtained assuming Standard Model production. The corresponding 95% confidence-level upper limits for the branching fractions of the $Z$ boson decays are $2.3\times {10}^{-6}$, $4.5\times {10}^{-6}$ and $(2.8,1.7,4.8)\times {10}^{-6}$, respectively.