Observation of W+W-gamma production in pp collisions at sqrt(s) = 13 TeV with the ATLAS detector and constraints on anomalous quartic gauge-boson couplings

This Letter reports the observation of W+W-gamma triboson production in 140 fb-1 of data collected by the ATLAS detector from proton-proton collisions at a centre-of-mass energy of sqrt(s) = 13 TeV at the LHC. Events with an opposite-charge emu pair, a high transverse-momentum photon, and significant missing transverse momentum are considered. The observed (expected) significance of the signal is 5.9 (6.0) standard deviations. The measured fiducial cross-section, defined for the W+W-gamma -> e+-mu-+nunubargamma final state is 6.2 +- 0.8 (stat.) +- 0.6 (sys.) fb, in good agreement with the Standard Model prediction of 6.1-0.7+1.0 fb. Constraints on the Wilson coefficients of 13 dimension-8 operators describing physics beyond the Standard Model through anomalous quartic gauge-boson couplings are derived using the effective field theory framework.