Search for exclusive Higgs and $Z$ boson decays to $\omega\gamma$ and Higgs boson decays to $K^{*}\gamma$ with the ATLAS detector

Searches for the exclusive decays of the Higgs boson to an ω meson and a photon or a $K^{⁎}$ meson and a photon can probe flavour-conserving and flavour-violating Higgs boson couplings to light quarks, respectively. Searches for these decays, along with the analogous Z boson decay to an ω meson and a photon, are performed with a pp collision data sample corresponding to integrated luminosities of up to 134 fb⁻¹ collected at $\sqrt{s}=13$ TeV with the ATLAS detector at the CERN Large Hadron Collider. The obtained 95% confidence-level upper limits on the respective branching fractions are $\mathcal{B}(H\to \omega \gamma )<5.5\times {10}^{-4}$, $\mathcal{B}(H\to K^{⁎}\gamma )<2.2\times {10}^{-4}$ and $\mathcal{B}(Z\to \omega \gamma )<3.9\times {10}^{-6}$. The limits for $H\to \omega \gamma$ and $Z\to \omega \gamma$ are 370 times and 140 times the Standard Model expected values, respectively. The result for $Z\to \omega \gamma$ corresponds to a two-orders-of-magnitude improvement over the limit obtained by the DELPHI experiment at LEP.