Title: The path to the search for rare event signals in XENON1T and XENONnT dark matter experiments
Description: A wide array of cosmological and astrophysical observations support the existence of dark matter. More precisely, temperature anisotropy measurements of the cosmic microwave background (CMB) estimate that the current dark matter mass density is about five times that of the visible Universe. However, the nature of dark matter is not yet understood, inspiring numerous theoretical candidates. One popular candidate is the weakly-interacting massive particles or WIMPs that interact with standard model particles on the electroweak scale and could have the correct relic abundance today. Experiments such as XENON1T and XENONnT are designed to search for WIMPs on Earth using the dual-phase liquid xenon Time Projection Chamber (LXeTPC) technology. The XENON1T experiment operated until Dec. 2018 and had made the world-leading upper limits for WIMP-nucleus interactions at the time. Its successor, the XENONnT experiment, has been commissioned since 2021 and has taken data for its first science run.

This thesis presents the commissioning data and the first science-run data analysis. This thesis describes an essential facet of the XENON1T and XENONnT experiments: how, step by step, the most elementary signals of single photons are reconstructed into events. Each event represents a particle interaction in the detector, including those from rare physical processes. This includes several technical developments with signal processing and simulation software that enable accurate reconstruction of signals and precisely evaluate the effect of various types of remaining miss-reconstruction. Furthermore, this thesis will present two analyses developed to search for rare events in XENON1T, only possible with an accurate and precise understanding of the event reconstruction. One is to search for ⁸𝐁 Solar neutrino events via 𝐂𝐄𝜈𝐍𝐒 process and low mass WIMPs by characterizing reconstruction efficiency and additional background at a lower energy threshold. The spin-independent DM-nucleus interaction is improved in the mass range between 3𝐆𝐞𝐕𝑐² and 11𝐆𝐞𝐕𝑐² by as much as an order of magnitude from the previous world-leading result, using data from the XENON1T experiment.

The other is the search for the neutrinoless double-beta decay at its 𝑄-value, 𝑄_𝛽𝛽 = (2457.83$\pm$0.37)\,keV. The analysis demonstrated that the relative energy resolution at one 𝝈/𝝁 is as low as (0.80±$0.02) % in its one-ton fiducial mass, and for single-site interactions at 𝑄_𝛽𝛽, a world-leading resolution in 𝐋𝐗e experiment that enhance the experimental sensitivity to the neutrinoless double-beta decay events.