Observation of the triboson process $pp \rightarrow W^\pm W^\mp \gamma$ and limits on anomalous quartic gauge couplings with the ATLAS detector

This thesis presents a search for evidence of $W^\pm W^\mp \gamma$ production from p−p scattering with $\sqrt{s}$ = 13 TeV at the Large Hadron Collider using 140 fb$^{−1}$ of integrated luminosity. The case where the $W$ bosons decay to opposite-flavor light leptons is considered, as other decay channels are dominated by backgrounds. Monte Carlo simulations are used to estimate the contributions of the $W^\pm W^\mp \gamma$ process as well as various background processes to the $e^\pm \mu^\mp \gamma$ channel. The contribution to the $e^\pm \mu^\mp \gamma$ channel of processes with a misidentified or non-prompt photon in the final state are estimated with data-driven methods. A machine learning algorithm trained on Monte Carlo simulations is used to further increase the purity of $W^\pm W^\mp \gamma$ events in the dataset. A maximum likelihood fit to the binned distribution of the machine learning discriminant is performed to determine the best-fit value of the $W^\pm W^\mp \gamma$ contribution to the $e^\pm \mu^\mp \gamma$ channel. From this, the expected $W^\pm W^\mp \gamma$ production cross section in a fiducial region is determined to be $10.5^{17\%}_{−11\%}$ (theory) $\pm$ 15% (experiment) fb. The observed cross section in the fiducial region will be measured after the $e^\pm \mu^\mp \gamma$ dataset is unblinded. Additionally, to study potential deviations from the Standard Model prediction in the $e^\pm \mu^\mp \gamma$ channel, the ATLAS run-2 dataset is used to set upper and lower limits at the 95% confidence level on 13 Wilson coefficients of an effective field theory extending the Standard Model with dimension-8 operators. Two methods to restore unitarity to the effective field theory are investigated: the clipping method as well as a dipole form factor model. The dependence of the Wilson coefficients’ expected upper and lower limits on these two methods’ parameters are presented.