Wayfinding: Searching for hints of new physics using a weakly-supervised anomaly search and a measurement of high-$p_\mathrm{T}$ $t\bar{t}Z$ production with the ATLAS experiment

Abstract

Efforts to discover a convincing discrepancy between experimental results and the Standard Model of particle physics have, up to this point, been unsuccessful. However, the Standard Model is known to be incomplete, and many theories of physics beyond the Standard Model predict new physics at the TeV scale or higher. This thesis presents two analyses designed to aid the search for new physics using proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider. The first, a weakly-supervised anomaly search in dijet events produced at a center-of-mass energy of $\sqrt{s} = \SI{13}{TeV}$, used novel machine learning techniques to search for resonances that clustered in high-dimensional phase space and manifested as bumps in the dijet invariant mass spectrum. The search was model-agnostic, insofar as it did not target any specific model of new physics and relied minimally on Standard Model simulation. No significant excess above the expected background was observed, though state-of-the-art limits were set on a wide variety of benchmark models. This analysis was the first weakly-supervised search in ATLAS to utilize more than one training feature, and the techniques developed in the search will provide a starting point for future anomaly searches. The second analysis was a measurement of $t\bar{t}Z$ production in final states with two leptons at a center-of-mass energy of $\sqrt{s} = \SI{13.6}{TeV}$. The measurement targeted the region of phase space in which the top quarks were produced with high transverse momentum. The top quarks were reconstructed using a jet reclustering technique. The production rate was measured inclusively, and differentially in 9 observables chosen to maximize sensitivity to the Standard Model effective field theory. This measurement was the first $t\bar{t}Z$ measurement at $\sqrt{s} = \SI{13.6}{TeV}$, and the first to target the high-$p_{\mathrm{T}}$ top quark regime. The inclusive measurement was found to be in agreement with the Standard Model prediction, but statistically-significant discrepancies were observed in the differential measurements. In particular, the $Z$ boson was found to have higher momentum than expected, and the top quarks lower. The results were qualitatively consistent with previous global fits to the Standard Model effective field theory.