Observation of WW$\gamma$ production and constraints on Higgs couplings to light quarks in proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$

The first observation of WW$\gamma$ production with leptonic final states in proton-proton collisions at a center-of-mass energy of 13 $\mathrm{TeV}$ is presented. A data set corresponding to an integrated luminosity of $ 138~\mathrm{fb}^{-1}$ collected by the CMS experiment at the LHC in 2016-2018 is used. 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 $\sigma = 6.0 \pm 1.7~\mathrm{fb}$, in good agreement with the next-to-leading order QCD prediction. The analysis is also extended to search for the associated production of a Higgs boson and a photon where the Higgs boson decays to W$^{+}$W$^{-}$, which is sensitive to the Higgs coupling to light quarks (the c, s, u, and d quarks), and limits on these couplings are reported at $95\%$ confidence level.