Observation of $WW\gamma $ Production and Search for $H\gamma $ Production in Proton-Proton Collisions at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$

The observation of $WW\gamma $ production in proton-proton collisions at a center-of-mass energy of 13 TeV with an integrated luminosity of $138\text{}\text{}{\mathrm{fb}}^{-1}$ is presented. The observed (expected) significance is 5.6 (5.1) standard deviations. Events are selected by requiring exactly two leptons (one electron and one muon) of opposite charge, moderate missing transverse momentum, and a photon. The measured fiducial cross section for $WW\gamma $ is $5.9±0.8\left(\mathrm{stat}\right)±0.8\left(\mathrm{syst}\right)±0.7\left(\mathrm{modeling}\right)\text{}\text{}\mathrm{fb}$, in agreement with the next-to-leading order quantum chromodynamics prediction. The analysis is extended with a search for the associated production of the Higgs boson and a photon, which is generated by a coupling of the Higgs boson to light quarks. The result is used to constrain the Higgs boson couplings to light quarks.