Keywords: Dark Matter, neutrinoless double beta decay, XENONnT, background, uncertainties. Liquid Xenon

Abstract: Dark Matter (DM) and Neutrinoless double beta decay (0νββ) have potential to reveal new physics beyond the Standard Model. The thesis is done in the framework of the XENONnT underground ultra-low background experiment. XENONnT (at the INFN LNGS in Italy) has a great sensitivity to elusive signals.
One part of this research is the calibration process of the XENONnT detector, which responds differently to nuclear and electronic recoils based on the interacting particle and energy range (keV to MeV). Using a variety of calibration sources, this study is focused on the detector's response, ensuring accurate signal identification.
Additionally, extensive background characterization and modeling – particularly from detector materials and surrounding environment – is essential for isolating true signals from noise. This thesis contributes by the background characterization, thus improving data interpretation for rare event searches.
This work advances XENONnT’s capabilities in the search for DM and 0νββ, aiding the broader quest for new physics. By developing improved approaches to background reduction, calibration, and signal analysis, this research supports the ongoing exploration of fundamental questions in physics and enhances our understanding of the universe.