Top Quark Physics at ATLAS: From Cross-Section Measurements to Search for New Particles

This thesis presents experimental studies of the top quark within the ATLAS experiment at the Large Hadron Collider (LHC). The top quark, with its mass of 172.5 GeV, is the heaviest known elementary particle and possesses unique properties: it has the strongest coupling to the Higgs boson among all SM particles and decays before hadronization, allowing its study as an almost free quark. Two complementary physics analyses are presented in this thesis. First, the measurement of the single top quark production cross section in the t-channel at a centre-of-mass energy of √s = 5.02 TeV, using pp collision data corresponding to an integrated luminosity of 255 pb−1 is detailed. After event selections a Boosted Decision Tree is employed for signal-to-background discrimination, and a Profile Likelihood fit is used to extract the total t-channel cross-section, the individual top- and antitop-quark production cross-sections, and their ratio. The t-channel single top quark process is observed for the first time at √s = 5.02 TeV with a significance of 6.1σ. The inclusive cross-section for the t-channel single-top production is measured to be σ(tq + ̄tq) = 27.1+4.4 −4.1 (stat.) +4.4 −3.7 (syst.) pb, while the individual cross-sections are measured to be σ(tq) = 19.8+3.9 −3.1 (stat.) +2.9 −2.2 (syst.) pb and σ( ̄tq) = 7.3+3.2 −2.1 (stat.) +2.8 −1.5 (syst.) pb respectively. The ratio between top- and antitop-quark production is found to be Rt = 2.73+1.43 −0.82 (stat.) +1.01 −0.29 (syst.). All measurements are in good agreement with the Standard Model (SM) predictions. Second, a search for new heavy resonances decaying to top-antitop quark pairs is conducted using the full Run 2 dataset, consisting of 140 fb−1 if pp collisions at √s = 13 TeV. The analysis targets three specific Beyond the Standard Model scenarios: a leptophobic Z′ boson from Top-colour-assisted technicolour, and two variants of Randall-Sundrum models predicting Kaluza-Klein excitations of the graviton and gluon. The search is performed in the semileptonic decay channel, targeting both re- solved and boosted topologies, implementing a specialized overlap-removal technique to enhance the sensitivity at high invariant masses. No significant deviation from the SM prediction is observed. Exclusion limits are set on the production cross-section times branching ratio for the considered signal models, obtaining a higher sensitivity in the extracted limits with respect to the previous ATLAS and CMS searches in the same decay channel. The thesis also includes technical contributions to the ATLAS collaboration combined performance effort, focusing on electron identification in environments with nearby hadronic jets. This study examines the electron identification process and its efficiencies in challenging scenarios where electron showers overlap with jet constituents. This work contributes to the development of current and especially future physics analyses in the ATLAS collaboration.

This thesis presents experimental studies of the top quark within the ATLAS experiment at the Large Hadron Collider (LHC). The top quark, with its mass of 172.5 GeV, is the heaviest known elementary particle and possesses unique properties: it has the strongest coupling to the Higgs boson among all SM particles and decays before hadronization, allowing its study as an almost free quark. Two complementary physics analyses are presented in this thesis. First, the measurement of the single top quark production cross section in the t-channel at a centre-of-mass energy of √s = 5.02 TeV, using pp collision data corresponding to an integrated luminosity of 255 pb−1 is detailed. After event selections a Boosted Decision Tree is employed for signal-to-background discrimination, and a Profile Likelihood fit is used to extract the total t-channel cross-section, the individual top- and antitop-quark production cross-sections, and their ratio. The t-channel single top quark process is observed for the first time at √s = 5.02 TeV with a significance of 6.1σ. The inclusive cross-section for the t-channel single-top production is measured to be σ(tq + ̄tq) = 27.1+4.4 −4.1 (stat.) +4.4 −3.7 (syst.) pb, while the individual cross-sections are measured to be σ(tq) = 19.8+3.9 −3.1 (stat.) +2.9 −2.2 (syst.) pb and σ( ̄tq) = 7.3+3.2 −2.1 (stat.) +2.8 −1.5 (syst.) pb respectively. The ratio between top- and antitop-quark production is found to be Rt = 2.73+1.43 −0.82 (stat.) +1.01 −0.29 (syst.). All measurements are in good agreement with the Standard Model (SM) predictions. Second, a search for new heavy resonances decaying to top-antitop quark pairs is conducted using the full Run 2 dataset, consisting of 140 fb−1 if pp collisions at √s = 13 TeV. The analysis targets three specific Beyond the Standard Model scenarios: a leptophobic Z′ boson from Top-colour-assisted technicolour, and two variants of Randall-Sundrum models predicting Kaluza-Klein excitations of the graviton and gluon. The search is performed in the semileptonic decay channel, targeting both re- solved and boosted topologies, implementing a specialized overlap-removal technique to enhance the sensitivity at high invariant masses. No significant deviation from the SM prediction is observed. Exclusion limits are set on the production cross-section times branching ratio for the considered signal models, obtaining a higher sensitivity in the extracted limits with respect to the previous ATLAS and CMS searches in the same decay channel. The thesis also includes technical contributions to the ATLAS collaboration combined performance effort, focusing on electron identification in environments with nearby hadronic jets. This study examines the electron identification process and its efficiencies in challenging scenarios where electron showers overlap with jet constituents. This work contributes to the development of current and especially future physics analyses in the ATLAS collaboration.