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