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Abstract

Within the context of the XENONnT experiment, this thesis presents contributions to low-energy electronic recoil (ER) searches and characterizations, advancing the quest for physics beyond the Standard Model (BSM).

Analysis of 1.16 tonne-years of XENONnT data from the first science run achieves a record-low ER background rate of (15.8 ± 1.3) events/(tonne·year·keV) in the 1 to 30 keV range, a 5-fold reduction over XENON1T. Calibration with the 2.82 keV 37Ar K-shell line establishes an accurate detector response in the critical few-keV region. The findings establish world-leading laboratory upper limits on solar axions, bosonic dark matter, and new neutrino physics. Specifically, the limits on new neutrino physics include those on non-standard interactions with vector and scalar mediators, as well as on an enhanced magnetic moment and millicharge. These results effectively rule out a BSM explanation for the previously observed XENON1T low-energy excess.

This work also reports the first calibration and observation of sub-keV ERs in liquid xenon (LXe), detecting the 0.27 keV 37Ar L-shell decay with 11.9σ significance. This is realized by combining a lowered energy threshold, resulting in a signal acceptance increase by a factor of about 66, with robust data-driven accidental coincidence background modeling and an improved suppression strategy. Validated full-chain simulations are used to understand sub-keV detector response and to determine the photon yield. From 37Ar L-shell events, a scintillation photon yield of PY = 2.96 ± 0.08 (stat.) +0.57/-0.33 (syst.) γ/keV is measured, providing essential data for sub-keV ER emission modeling.

Looking towards the next generation of LXe observatories like XLZD, this thesis includes developments for the automated rare gas mass spectrometer (Auto-RGMS), an instrument combining a cryogenic gas chromatography stage and a mass spectrometer for ultra-sensitive parts-per-quadrillion (ppq) quantification of krypton traces in xenon gas. Controlling and characterizing the beta-decaying 85Kr isotope is critical, as it represents a significant ER background in low-energy analyses. By using the new adsorbent, HayeSep Q, a 12-fold improvement in chromatographic resolution is achieved. This enhancement enables processing of much larger samples and helps achieve the designed few-ppq detection limit needed for XLZD's future high-precision measurements of solar pp neutrinos and the weak mixing angle.

Furthermore, sensitivity projections demonstrate that the newly validated sub-keV ER analysis framework can enhance XLZD's searches for BSM physics, improving sensitivity to neutrino electromagnetic properties by up to 19% and to boosted dark matter by up to a factor of two, thereby maximizing the scientific output of future multi-ten-tonne scale LXe experiments.