Search for exclusive Higgs and $Z$ boson decays to $\phi\,\gamma$ and $\rho\,\gamma$ with the ATLAS Detector

A search for the exclusive decays of the Higgs and $Z$ bosons to a $\phi$ or $\rho$ meson and a photon is performed with a $pp$ collision data sample corresponding to an integrated luminosity of up to $35.6\,\text{fb}^{-1}$ collected at $\sqrt{s}=13\,\text{TeV}$ with the ATLAS detector at the CERN Large Hadron Collider. These decays have been suggested as a probe of the Higgs boson coupling to light quarks. No significant excess of events is observed above the background, in agreement with the Standard Model expectations. Upper limits at 95\% confidence level on the branching fractions of the Higgs boson decays to $\phi\,\gamma$ and $\rho\,\gamma$ of $4.8\times10^{-4}$ and $8.8\times10^{-4}$, respectively, are obtained. The corresponding 95\% confidence level upper limits for the $Z$ boson decays are $0.9\times10^{-6}$ and $25\times10^{-6}$ for $\phi\,\gamma$ and $\rho\,\gamma$, respectively.