Searches for exclusive Higgs boson decays into And Boson Decays Into And In Collisions at with the ATLAS detector

Searches for exclusive decays of the Higgs boson into ${D}^{⁎}\gamma $ and of the Z boson into ${D}^{0}\gamma $ and ${K}_{s}^{0}\gamma $ can probe flavour-violating Higgs boson 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 $\sqrt{s}=13\phantom{\rule{0.25em}{0ex}}\text{TeV}$ between 2016–2018 with the ATLAS detector at the CERN Large Hadron Collider. In the ${D}^{⁎}\gamma $ and ${D}^{0}\gamma $ channels, the observed (expected) 95% confidence-level upper limits on the respective branching fractions are $B\left(H\to {D}^{⁎}\gamma \right)<1.0\left(1.2\right)×{10}^{-3}$, $B\left(Z\to {D}^{0}\gamma \right)<4.0\left(3.4\right)×{10}^{-6}$, while the corresponding results in the ${K}_{s}^{0}\gamma $ channel are $B\left(Z\to {K}_{s}^{0}\gamma \right)<3.1\left(3.0\right)×{10}^{-6}$.