The observation of $WW\gamma$ production in proton-proton collisions at a center-of-mass energy of 13 TeV with an integrated luminosity of $138{\mathrm{fb}}^{-1}$ 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 $5.9\pm 0.8(\mathrm{stat})\pm 0.8(\mathrm{syst})\pm 0.7(\mathrm{modeling})\mathrm{fb}$, 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.

• Received 8 October 2023
• Accepted 31 January 2024

DOI:https://doi.org/10.1103/PhysRevLett.132.121901

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