2025 Theses Doctoral

Search for Dark Matter with the XENONnT Experiment: Knowns and Unknowns

Xu, Zihao

This thesis presents the search for Weakly Interacting Massive Particles (WIMPs) with the XENONnT experiment, a dual-phase liquid xenon time projection chamber (TPC) operated with multiple subsystems for shielding, purification, and active vetoing. The experiment achieved an unprecedentedly low electronic-recoil background, primarily through suppression of 214Pb decays by radon distillation system. A detailed reconstruction and correction framework was developed to account for detector spatial and temporal non-uniformity. A bayesian fitting framework was developed to constrain the detector response to nuclear and electronic recoils using calibration sources.

While the underlying microphysics of scintillation and ionization at low energies remains an open problem, the statistical inference framework, combined with a strict blinding strategy, enabled a robust analysis of the WIMP search region. After final unblinding, no significant excess above background was observed, and the results were interpreted as upper limits on the spin-independent WIMP-nucleon cross section, setting strong constraints on dark matter models.