Supersymmetric Beasts and Where to Find Them: From Novel Hadronic Reconstruction Methods to Search Results in Large Jet Multiplicity Final States at the ATLAS Experiment

Supersymmetry (SUSY) is one of the most appealing extensions of the Standard Model (SM), predicting the existence of a partner for each particle of the SM introducing a fermion-boson symmetry. This theory offers an answer to many of the current open questions of physics, such as the origin of Dark Matter, the Higgs boson mass hierarchy problem and the unification of elementary forces at high energy. SUSY particles could manifest themselves in different ways in the proton-proton (p-p) collisions of the Large Hadron Collider (LHC), allowing the design of dedicated searches that explore these characteristic signatures. This work focuses on a search carried out by the ATLAS collaboration targeting SUSY particles that decay via long cascades, leading to final states with zero leptons, large jet multiplicities (8-12 jets) and moderate Missing Transverse Momentum ($E_T^\text{miss}$). The analysis strategy and results obtained using $139\;\text{fb}^{-1}$ of $\sqrt{s}=13\;\text{TeV}$ data collected by the ATLAS experiment during the complete Run 2 of the LHC are presented. No significant excess above the SM prediction was found, and 95% CL exclusion limits were set on different simplified SUSY models. This search is the first ATLAS analysis to use Particle Flow jet and $E_T^\text{miss}$ reconstruction, a technique that combines information from different sub-detectors in order to improve the reconstruction precision and thus the sensitivity to new physics. This work describes the development of the ATLAS Particle Flow $E_T^\text{miss}$, a novel $E_T^\text{miss}$ reconstruction technique commissioned during the second Run of the LHC. Improvements of this quantity with respect to standard calorimeter-based $E_T^\text{miss}$ reconstruction techniques are shown. This work also presents novel ideas for the mitigation of the neutral component of the multiple p-p interactions of the LHC (pileup). These studies were made in the context of Particle Flow using multivariate analysis techniques. In future, these ideas might further improve the ATLAS Particle Flow stability to pileup. In 2026, the LHC will be upgraded to the High-Luminosity LHC (HL-LHC), being able to deliver five times more luminosity than the original LHC design value. This will result in much higher pileup and data rates, exceeding the current design features of the ATLAS detector. The upgrade of the ATLAS Trigger and Data Acquisition system will be essential to realise the HL-LHC physics program. In this context, a novel Hardware Track Trigger (HTT) will be included for reducing the trigger rates and increase the sensitivity to new physics. This work presents a set of studies and techniques for the suppression of pileup using online HTT tracks in the context of multi-jet and $E_T^\text{miss}$ triggers. These studies show significant reductions in the rates of these triggers, leading to better acceptance of important signals such as $ZH \rightarrow b\overline{b}\nu \overline{\nu}$ and $HH\rightarrow b\overline{b}b\overline{b}$.