This thesis describes in detail a search for weakly interacting massive particles as possible dark matter candidates, making use of so-called mono-jet events. It includes a detailed description of the run-1 system, important operational challenges, and the upgrade for run-2. The nature of dark matter, which accounts for roughly 25% of the energy-matter content of the universe, is one of the biggest open questions in fundamental science. The analysis is based on the full set of proton-proton collisions collected by the ATLAS experiment at the Large Hadron Collider at √s = 8 TeV. Special attention is given to the experimental challenges and analysis techniques, as well as the overall scientific context beyond particle physics. The results complement those of non-collider experiments and yield some of the strongest exclusion bounds on parameters of dark matter models by the end of the Large Hadron Collider run-1. Details of the upgrade of the ATLAS Central Trigger for run-2 are also included.
Abstract: Vector boson scattering is one of the recent remarkable observations at the Large Hadron Collider. The longitudinal polarization modes of the massive vector bosons are strongly tied to the electroweak symmetry breaking mechanism. With the Standard Model predicted Higgs boson playing a crucial role in regularizing the scattering amplitude of these longitudinally polarized bosons, vector boson scattering is a pivotal process in experimentally probing the symmetry breaking mechanism. A golden channel for measuring vector boson scattering at the collider is the electroweak production of two $W$ bosons with the same electric charges. Owing to its largest quark- to gluon- induced production ratio among other di-boson combinations, the process was also the first target of the ATLAS vector boson scattering program, with evidence made in 2014 and observation in 2019. This thesis presents the first measurement of the differential cross-section with the ATLAS experiment at $\sqrt{s}=13$\,TeV using 139\,fb$^{-1}$ datasets of proton-proton collisions. The process is studied in the leptonic decay channels of the $W$ bosons, effectively suppressing many Standard Model backgrounds. The process $pp \rightarrow l^{\pm} \nu l^{\pm} \nu jj $ is measured with the final state consisting of two leptons of like charges, two jets, and missing transverse energy. The characteristic vector boson scattering signature of two tagging jets, with a large di-jet invariant mass, separated by large angles, is used to tag electroweak-induced production. A combination of Monte Carlo-based predictions and data-driven approaches is used to estimate the various backgrounds. A statistical model of profile likelihood is used to constrain the background predictions and reduce the uncertainties following which the events are unfolded, and the cross-section is extracted. The fiducial differential cross-sections are measured in the leptonic channel as a function of several kinematic variables and are found to be consistent with the Standard Model predictions within uncertainties. An experimental precision of 10.2\% is achieved for the fiducial cross-section, and the measurement is unprecedented in precision and granularity for the process. The integrated fiducial cross-section is $3.51 \pm 0.27\,\text{(stat)}\,\pm 0.23\,\text{(syst)}\,\text{fb}$ and agrees with the leading order prediction of $2.97^{+0.28}_{-0.24}\,\text{fb} $ simulated by MadGraph+Herwig7 within uncertainties
