Probing Electroweak Gauge Boson Scattering with the ATLAS Detector at the Large Hadron Collider

Electroweak gauge bosons as central components of the Standard Model of particle physics are well understood theoretically and have been studied with high precision at past and present collider experiments. The electroweak theory predicts the existence of a scattering process of these particles consisting of contributions from triple and quartic bosonic couplings as well as Higgs boson mediated interactions. These contributions are not separable in a gauge invariant way and are only unitarized in the case of a Higgs boson as it is described by the Standard Model. The process is tied to the electroweak symmetry breaking which introduces the longitudinal modes for the massive electroweak gauge bosons. A study of this interaction is also a direct verification of the local gauge symmetry as one of the fundamental axioms of the Standard Model. With the start of the Large Hadron Collider and after collecting proton-proton collision data with an integrated luminosity of $20.3\;\mathrm{fb}^{-1}$ at a center-of-mass energy of $\sqrt{s} = 8\;\mathrm{Te\kern -0.1em V}$ with the ATLAS detector, first-ever evidence for this process could be achieved in the context of this work. A study of leptonically decaying $W^{\pm}W^{\pm}jj$, same-electric-charge diboson production in association with two jets resulted in an observation of the electroweak $W^{\pm}W^{\pm}jj$ production, inseparably comprising $W^{\pm}W^{\pm}\to W^{\pm}W^{\pm}$ electroweak gauge boson scattering contributions, with a significance of 3.6 standard deviations. The measured production cross section is in agreement with the Standard Model prediction. In the course of a study for leptonically decaying $WZ$ productions, methods for background estimation, the extraction of systematic uncertainties and cross section measurements were developed. They were extended and applied to the $WZjj$ final state whereof the purely electroweakly mediated contribution is intrinsically tied to the scattering of all Standard Model electroweak gauge bosons: $W\gamma\to WZ$ and $WZ\to WZ$. Three charged leptons and a neutrino from the decay of the final state bosons allow inferences about the scattering process. A distinct signature is provided by the two accompanying tagging jets as remnants of the incoming quarks radiating the initial electroweak gauge bosons. The cross section of the electroweak $WZjj$ production was measured to $\sigma_{\mathrm{fiducial}}^{\mathrm{observed}} = \left(0.63\;^{+0.32}_{-0.28}\;(\mathrm{stat.})\;^{+0.41}_{-0.24}\;(\mathrm{syst.})\right)\;\mathrm{fb}$ and was found to be consistent with the Standard Model prediction at next-to-leading order in perturbative quantum chromodynamics, $\sigma_{\mathrm{fiducial}}^{\mathrm{theory}} = \left(0.31\;^{+ 0.03 }_{- 0.05 }\right)\;\mathrm{fb}$. Unfolded differential cross sections of kinematic variables sensitive to models of new physics were derived. Anomalous quartic electroweak gauge couplings are introduced as dimensionless coupling parameters of additional operators within an effective field theory approach. Constraints on the parameters of operators with dimension eight were set employing a unitarization prescription based on form factors.