Search for exclusive Higgs and Boson Decays to and Higgs boson decays to 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 $B\left(H\to \omega \gamma \right)<5.5×{10}^{-4}$, $B\left(H\to {K}^{⁎}\gamma \right)<2.2×{10}^{-4}$ and $B\left(Z\to \omega \gamma \right)<3.9×{10}^{-6}$. 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.