First Directional Search for Magnetic Inelastic Dark Matter with XENONnT

The XENONnT Experiment, operated at the Laboratori Nazionali del Gran Sasso, is designed to search for Dark Matter (DM) by detecting recoils between DM particles in the galactic halo and target xenon atoms. To date, the majority of efforts have focused on searching for elastic recoils between DM and xenon nuclei, as predicted by the Weakly Interacting Massive Particle (WIMP) model. The lack of a confirmed WIMP detection has motivated increased interest in alternative DM models in recent times. In this dissertation, we present a novel analysis framework developed specifically for the search for Magnetic Inelastic Dark Matter (MiDM) signals in the XENONnT detector. MiDM proposes the existence of two closely spaced DM mass states that couple to the Standard Model through a magnetic dipole operator. This leads to a new set of characteristic observable signatures, including inelastic nuclear recoils and delayed DM decays. We combine two distinct signal topologies in the search: single-site events, in which the DM decay occurs outside the XENONnT detector, and double-site events, in which it occurs within the detector. We identify and model the signal and background event sources in each channel and develop a tailored combined statistical inference framework to carry out the search. We exploit the properties of MiDM double-site signals to define selection criteria that reduce background levels to near zero, thereby achieving sensitivity to a single detected event. Particular emphasis is placed on the joint modeling of spatial and temporal correlations between the scattering and decay interaction sites, which is essential for discriminating MiDM signals from rare background processes. We find no evidence for MiDM in our dataset, allowing us to place world-leading constraints on the magnetic dipole moment of DM. We also highlight potential improvements and future extensions for searches of inelastic DM in liquid xenon time projection chambers.