Measurement and interpretation of inclusive $W\gamma$ production in proton-proton collisions at $\sqrt{s}=13$ TeV using the ATLAS detector

Differential cross-section measurements are presented for the production of a $W$ boson in association with a photon. The analysis is performed using proton–proton collision data collected by the ATLAS experiment at $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 140~fb${}^{-1}$. The differential cross sections are measured in the $W\gamma \to \mathrm{\ell }\nu \gamma$ decay channel ($\mathrm{\ell }=e,\mu$) as a function of 16 observables. Collectively, these observables probe the kinematic properties of the $W\gamma$ system, the radiation amplitude zero effect predicted for the $W\gamma$ final state, the polarisation of the $W$ boson, the charge conjugation and parity structure of the $WW\gamma$ triple gauge coupling, and the parton distribution functions of the proton. The data are corrected for the effects of detector inefficiency and resolution and are sufficiently precise that they can be used to distinguish between different state-of-the-art theoretical predictions provided by SHERPA, MADGRAPH5_aMC@NLO, and GENEVA. The differential cross sections are used to search for anomalous weak-boson self-interactions induced by dimension-six operators within an effective field theory. For CP-odd operators, dedicated detector-corrected observables based on the outputs of neural networks are found to be particularly sensitive to the interference between the Standard Model and dimension-six scattering amplitudes. Constraints are placed on the Wilson coefficients of the ${\mathcal{o}}_W$, ${\mathcal{o}}_{HWB}$, ${\mathcal{o}}_{\tilde{W}}$ and ${\mathcal{o}}_{H\tilde{W}B}$ operators in the effective field theory. The sensitivity to the ${\mathcal{o}}_{H\tilde{W}B}$ operator is improved by a factor of 2.5 compared to previous measurements in other final states.