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Abstract

Many astrophysical and cosmological observations point to the existence of a massive, nonluminous and non-baryonic component of matter that is called dark matter (DM). The XENON Dark Matter project aims for the direct detection of DM in the form of weakly interacting massive particles (WIMPs), which are one of the most compelling candidates for particle DM. Utilizing the concept of a dual-phase xenon time projection chamber (TPC), the XenonnT experiment is the latest installment in the Xenon experiment series, and was designed as a fast upgrade of the predecessor experiment Xenon1T. With an increased liquid xenon (LXe) mass of 5.9 tonnes and novel subsystems, the XenonnT detector has reached an unprecedented purity in both electro-negative and radioactive radon contaminations, and has been taking data since 2021. In the first WIMP search results obtained with a blind analysis of the 1.1 tonneyears of exposure taken in XenonnT’s first science run, SR0, stringent upper limits on the WIMP-nucleon interaction cross-section were set, with a minimum of 2.58 × 10⁻⁴⁷ cm² for a WIMP mass of 28 GeV/c² at a 90 % confidence level.

This work is centered around the XenonnT experiment and its first WIMP search results. During the construction phase of the experiment, the photomultiplier tubes (PMTs) used to detect the scintillation light induced by particle interactions in the TPC were characterized, selected and installed. Their performance in the XenonnT TPC was analyzed and monitored throughout detector commissioning and the first science run, focussing on the identification of vacuum degradation and afterpulse behavior. For the first WIMP search analysis, the detector’s response to nuclear recoil interactions expected for a WIMP signal was modeled, using neutron calibration data. Additionally, the small-scale Heidelberg Xenon (HeXe) TPC was used to investigate the response of LXe to γ-ray interactions under the same experimental conditions as in the XenonnT detector.