Commissioning of the world’s first water Cherenkov neutron veto and first WIMP dark matter search results of the XENONnT experiment

Abstract

A rich number of astronomical and cosmological observations suggest the existence of a massive, non-luminous, and non-relativistic, matter component in the universe which is five times more abundant than baryonic matter and is commonly referred as to dark matter (DM). Although so far eluding from detection, one class of promising DM candidates are weakly interacting massive particles (WIMPs) which arise naturally from many beyond the Standard Model (BSM) theories. The XENON Dark Matter Project aims to directly detect WIMPs, and other kinds of rare event signals, by utilizing large-scale liquid xenon (LXe) dual-phase time projection chambers (TPCs). The newest generation of experiment, called XENONnT, utilizes a TPC with a total sensitive LXe mass of 5.9 t, and was designed as a fast upgrade of its predecessor XENON1T. In addition to its larger TPC, XENONnT was augmented with the world’s first water Cherenkov neutron veto (NV), which was mounted inside the already existing water Cherenkov muon veto water tank of XENON1T. Neutrons emitted by detector materials can undergo a single back-scatter inside the TPC producing a signal which is indistinguishable from WIMPs. The NV has the task to mitigate this potential threat for the scientific reach of the experiment by tagging these escaping neutrons through their delayed neutron capture on hydrogen. In the presented work, the results of the first weakly interacting massive particle (WIMP) search science run, called SR0, are discussed. SR0 features a blind analysis between 3.3 keV and 60.5 keV nuclear recoils energies with a total exposure of about 1.1 tonne-year, utilizing the lowest ever achieved electronic recoil background of (15.8 ± 1.3) events/(t · y · keV) in a LXe. No significant excess was found in the data, setting the lowest upper limit of 2.58 · 10−47 cm2 for spin-independent (SI) interactions of 28 GeV/c2 WIMPs at a 90% confidence level. These results have also been published in [Apr+23b] as part of the presented work. To obtain these results, this thesis discusses the commissioning of the XENONnT neutron veto (NV), and the calibration of its neutron tagging efficiency. The tagging efficiency was found to be (53.1±2.8)% which is the highest efficiency ever measured in a water Cherenkov detector. The efficiency of the NV, as well as the nuclear recoil (NR) response of the time projection chamber (TPC), were calibrated using tagged neutrons from an Americium-Beryllium (AmBe) neutron source. This technique was deployed for the first time in a liquid xenon (LXe) TPC. It enables a calibration of the NR response with high purity and a remaining pollution of less than 0.1 %. Further, the same calibration data was used to determine the thermal neutron capture cross section of hydrogen which was found to be 336.7 ± 0.4 (stat.)+2.0 −0.0 (sys.) mb. All these analyses are based on the data provided by XENONnT’s new processing framework called STRAXEN. As part of the lead developing team, the entire processing chain for the two veto systems of XENONnT was developed, and many additional tools have been implemented. Finally, to enhance the neutron tagging efficiency of the NV even further, the water inside the water tank is going to be doped with Gd-sulfate. As part of the presented work, different Gd-salt samples of the manufacturer Treibacher were analyzed regarding their suitability for the experiment.