Searches for exclusive Higgs boson decays into $D^{\ast }\gamma$ and $Z$ boson decays into $D^0\gamma$ and $K_s^0\gamma$ in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Searches for the exclusive decays of the Higgs boson into $D^{\ast }\gamma$ and of the $Z$ boson into $D^0\gamma$ and $K_s^0\gamma$ can probe flavour-violating Higgs and $Z$ boson couplings to light quarks. Searches for these decays are performed with a $pp$ collision data sample corresponding to an integrated luminosity of $136.3$ fb${}^{-1}$ collected at $\sqrt{s}=13$ TeV between 2016-2018 with the ATLAS detector at the CERN Large Hadron Collider. In the $D^{\ast }\gamma$ and $D^0\gamma$ channels, the observed (expected) 95$\mathrm{\%}$ confidence-level upper limits on the respective branching fractions are $\mathcal{B}(H\to D^{\ast }\gamma )<1.0(1.2)\times {10}^{-3}$, $\mathcal{B}(Z\to D^0\gamma )<4.0(3.4)\times {10}^{-6}$, while the corresponding results in the $K_s^0\gamma$ channel are $\mathcal{B}(Z\to K_s^0\gamma )<3.1(3.0)\times {10}^{-6}$.