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 $e\mu$ 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 $W^{+}{W}^{-}\gamma \to e^{\pm }{\mu }^{\mp }\nu \bar{\nu }\gamma$ final state is 6.2 $\pm$ 0.8 (stat.) $\pm$ 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.