Ramírez García, Diego
[Author]
;
Schumann, Marc
[Degree supervisor];
Schumann, Marc
[Other];
Schumacher, Markus
[Other]Albert-Ludwigs-Universität Freiburg Fakultät für Mathematik und Physik
Simulating the XENONnT dark matter experiment: backgrounds and WIMP sensitivity
University thesis:
Dissertation, Universität Freiburg, 2022
Footnote:
Description:
Abstract: XENONnT is a dark matter direct detection experiment exploiting the dual-phase xenon time projection chamber technique. Located at the INFN Laboratori Nazionali del Gran Sasso, and with 5.9 t of instrumented liquid xenon, it is the upgraded version of XENON1T, which is up to now the world-leading experiment in the search for WIMPs. In this work we predict the experimental background and project the WIMP sensitivity of XENONnT.<br><br>The first part of this thesis summarizes the radioassay campaign carried out to select materials with low traces of radioactive contaminants for the detector construction, in order to achieve the low event rate regime required for WIMP searches. In the context of this effort the GeMSE (Germanium Material and meteorite Screening Experiment) detector has been utilized. This gamma spectrometer features an ultra-low background rate of (164 ± 2) counts/day in the 100-2700 keV region, hence it is idoneus for the XENONnT material selection campaign. The full characterization of this background is described, as well as potential ways for further reduction. In addition, the improved remote operation and simulations to evaluate the efficiency of the measured samples are also reported.<br><br>A detector model based on the Geant4 toolkit has been developed to evaluate the XENONnT performance, along with additional packages to convert the simulated energy deposits into data-like signals, based on detector design and early operation inputs. The Monte Carlo simulations pipeline is described in the second part of this thesis. Lastly, this framework is employed together with the radiopurity information from the previous section to estimate the XENONnT background rate and projected sensitivity in the search for WIMPs.<br><br>The expected average differential background rate in the energy region of interest, corresponding to (1, 13) keV and (4, 50) keV for electronic and nuclear recoils, amounts to (12.3 ± 0.6) (keV t y)⁻¹ and (2.2 ± 0.5)x10⁻³ (keV t y)⁻¹, respectively, in a 4 t fiducial mass. With the exposure goal of 20 t y, the expected sensitivity to spin-independent WIMP-nucleon interactions reaches a cross-section of 1.4x10⁻⁴⁸ cm² for a 50 GeV/c² mass WIMP at 90% confidence level, more than one order of magnitude beyond the current best limit, set by XENON1T. For a 50 GeV/c² WIMP with cross-sections above 2.6x10⁻⁴⁸ cm² (5.0x10⁻⁴⁸ cm²) the median XENONnT discovery significance exceeds 3σ (5σ). The expected sensitivity to the spin-dependent WIMP coupling to neutrons (protons) reaches 2.2x10⁻⁴³ cm² (6.0x10⁻⁴² cm²). With the commissioning phase already concluded, the XENONnT detector is currently taking science data