Few-electron signals and their implications in liquid xenon time projection chambers

The energy threshold of liquid xenon detectors is driven by the requirements of observing a scintillation signal as well as a large ionization signal. Observing both allows powerful background rejection, but limits the sensitivity below O(10 GeV/c²). Removing the requirement of having a scintillation signal, the threshold for light dark matter can be pushed lower. One limitation to the light dark matter search in XENON1T was single- and few-electron backgrounds that were not well understood. A dedicated analysis was performed to understand these backgrounds and event selections were developed to mitigate them.  This thesis presents details of the characterization and results from a search for light dark matter using only the single- and few-electron ionization signals in the XENON1T detector.

These liquid xenon detectors are leading in sensitivity to search for rare events. With various detector upgrades, XENONnT has improved sensitivity to low-energy interactions with signals as low as a single detected electron. This allows XENONnT to be able to detect neutrinos of all flavors from potential Galactic supernovae via coherent elastic neutrino-nucleus scattering (CEvNS). This thesis presents an overview of the capability of XENONnT to detect supernova neutrinos via CEvNS. This allows XENONnT to be the first direct detection dark matter experiment to directly participate in the SuperNova Early Warning System.