A precision measurement of fiducial and differential cross sections of $WW$ production with the atlas detector at $\sqrt{s}=13\,\mathrm{TeV}$

Measuring production cross sections of W-boson pairs (WW) at particle colliders provides an important test of the predictions of Standard Model of particle physics in both perturbative quantum chromodynamics and electroweak domains. In this thesis, fiducial and differential cross-section measurements of WW production are presented. A dataset recorded with the ATLAS detector between 2015 and 2018 in proton-proton collisions at a center-of-mass energy of sqrt(s)=13 TeV at the Large Hadron Collider is analyzed, corresponding to an integrated luminosity of 140 inverse femtobarns. The event selection targets leptonic decays into an electron and a muon of opposite electric charge. In contrast to many previous measurements that enhance the WW signal purity by vetoing hadronic jets in the final state, the first measurement of the WW production cross sections using a fully jet-inclusive selection is presented in this thesis. Contributions from top-quark and lepton misidentification backgrounds are estimated using data-driven techniques. The latter relies on the definition of lepton selection criteria targeting non-prompt lepton contributions. An accurate modeling of prompt leptons fulfilling these selection requirements is ensured by performing a dedicated calibration using the tag-and-probe method. The associated uncertainties are drastically reduced, no longer being a limiting factor in the precision of WW cross-section measurements. The fiducial WW cross section is determined in a maximum-likelihood fit with an uncertainty of 3.1%. The measurement is extrapolated to the full phase space, resulting in a total WW cross section of 127+-1(stat.)+-4(syst.) picobarns, providing the most precise measurement of the WW production cross section achieved in hadron-hadron collisions to date. Differential cross sections are measured as a function of twelve observables describing the kinematics of leptons, jets, and the missing transverse energy of the WW system. State-of-the-art theory predictions are in excellent agreement with the reported measurements. Differential distributions at reconstruction level are used to constrain anomalous electroweak gauge boson self-couplings and interactions between the Higgs scalar field, leptons, and quarks in the framework of a dimension-six effective field theory. To address the numerous challenges in environments of high instantaneous luminosity expected during the operation of the LHC Run 3 and High-Luminosity LHC, upgrades of the experimental detectors were carried out between 2019 and 2022. These involved the replacement of the innermost muon chambers in the ATLAS detectors endcaps with the New Small Wheels (NSWs). A new facility to test small-strip Thin Gap Chambers (sTGC), one of the main technologies of the NSW, has been commissioned in Freiburg. Design, installation, and validation of the gas and high-voltage systems necessary for operating a sTGC prototype under nominal conditions of LHC data-taking are reported in this thesis.