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

Dark matter makes up about 27% of the universe, yet its true nature remains one of the biggest open questions in physics. This seminar presents a search for axinos—hypothetical particles that arise in theoretical models where both supersymmetry and axions are present. In particular, the DFSZ axion model, proposed to solve the strong CP problem, can be extended to include supersymmetric extensions to the Standard Model, predicting a stable DFSZ axino as a viable dark matter candidate. In such models, long-lived neutralinos may decay into axinos and Standard Model particles, producing distinctive displaced vertices with high mass and multiple tracks, along with significant missing transverse energy. The analysis uses 137 inverse femtobarns of high-energy proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider. It employs data-driven methods to estimate backgrounds and interprets the results in the context of the DFSZ axino model, where the axino is the lightest supersymmetric particle. Although no significant excess over the expected background was observed, we set stringent 95% confidence level exclusion limits on the production cross section as a function of the DFSZ axino mass and DFSZ axion mass. This work also supports ongoing upgrades to the ATLAS trigger system, particularly the global feature extractor (gFEX), which is designed to improve sensitivity to events with large missing energy and jets in future data-taking. These results enhance our ability to search for long-lived particles and bring us closer to uncovering the particle nature of dark matter.