Searches for heavy top partners with the ATLAS detector and irradiation studies of the Tile hadronic calorimeter

This dissertation presents two searches for heavy, vector-like partners of Standard Model top quarks in proton-proton collision data, with a center-of-mass energy of $\sqrt{s}=13$ TeV, collected by the ATLAS detector at the CERN Large Hadron Collider. The first analysis searches for pair production of vector-like quarks ($T$), using the 2015-2016 dataset, corresponding to 36.1 fb$^{-1}$ of integrated luminosity. The search primarily targets $T\to Ht$ and $T\to Zt$ decays, by analyzing events in the lepton+jets final state, characterized by a single isolated electron or muon, large missing transverse momentum, and multiple jets, as well as in the jets+$E_{\text{T}}^{\text{miss}}$ final state, containing zero leptons, a high multiplicity of jets, and large missing transverse momentum. The search exploits the high multiplicity of $b$-jets and hadronically decaying boosted top quarks and Higgs bosons, which are characteristic of signal events. The second search targets single $T$ production, using the full Run 2 dataset taken during 2015-2018, corresponding to 139 fb$^{-1}$ of integrated luminosity. The search analyzes events in final states with a single lepton with multiple jets and $b$-jets, also targeting $T\to Ht$ and $T\to Zt$ decays, and using tagging algorithms to identify both hadronically and leptonically decaying boosted objects. The presence of a forward jet, characteristic of the targeted signal, along with the multiplicity of jets, $b$-jets, and reconstructed boosted objects, is used to categorize the analyzed events. No significant excess above the Standard Model expectation is observed in either analysis, and the results are used to set 95% CL upper limits on the production of $T$ quarks. The first search excludes $T$-quark masses ranging from 0.99 TeV for any configuration of decay parameters, and up to 1.43 TeV for $Ht$ decays. The second search sets limits on the model-dependent single $T$ production mode, excluding a range of masses in different benchmark scenarios, between 1.27 TeV for unfavorable model parameters, up to the highest mass considered in the analysis of 2.1 TeV in the case of stronger coupling values. Additionally, studies are presented on the effect of accumulated irradiation during data taking on the Tile hadronic calorimeter in the ATLAS detector. These studies use events with minimal kinematic requirements in combination with measurements from other calibration systems to monitor and diagnose the behavior of different components of the calorimeter readout chain, as well as to analyze the long-term evolution of the performance of the scintillating tiles for extrapolation to upcoming data taking periods.