Shadows in the Detector: Searching for Axino Dark Matter as Missing Energy in ATLAS and Commissioning MET and Jet Triggers

This thesis presents a search for long-lived particles decaying to axinos using 137 inverse femtobarn of proton-proton collision data collected by the ATLAS experiment at 13 TeV. The analysis focuses on events featuring displaced vertices with high invariant mass and track multiplicity, accompanied by substantial missing transverse energy. Results are primarily interpreted in the context of DFSZ axino models, where the axino serves as the lightest supersymmetric particle and a potential dark matter candidate. Background estimation is performed using data-driven techniques, with systematic uncertainties carefully evaluated through multiple validation regions. No significant excess over the estimated background is observed, allowing exclusion limits to be set on the model parameters at 95% confidence level. This work also details contributions to the gFEX system — a component of the ATLAS Level-1 trigger upgrade that enhances detection capabilities for missing energy and jet signatures. These developments strengthen future searches for axinos and other phenomena involving missing transverse energy, advancing our understanding of dark matter and beyond Standard Model physics.