Astrophysical observations hint to the existence of two ingredients, dominating the evolution of the universe: Dark Energy and Dark Matter. Dark Energy is considered a uniform energy field, driving the expansion of the universe. Dark Matter might be a new particle, or other new physics, interfering with our understanding of gravity.
The analysis of galaxy movement in galaxy clusters indicated the existence of invisible matter; the first evidence for Dark Matter duped Dunkle Materie by Fritz Zwicky in the 1930s.
The observation of the movement of gas clouds and stars in (distant) galaxies reveals deviations from the expected velocities according to Kepler’s law and galaxy mass estimations using the observable, light emitting matter. The orbital velocities stay nearly constant, instead of decreasing with further distance from the core. Another indication for the existence of a significant amount of invisible matter, distributed far beyond the luminous matter of the galaxy.
Further strong hints are found in anisotropies of the cosmic microwave background. They are caused by density fluctuations of cold dark matter in the early universe short before recombination at about 380000 years after the Big Bang. These fluctuations, at the level of μK were observed and measured precisely by the satellite-based experiments WMAP and Planck. The fluctuations caused acoustic oscillations in the interstellar plasma, which were then frozen in shape at recombination. The relative size of the acoustic peaks of this characteristic oscillation pattern allows the computation of the composition of the universe, which is found to be 4.9 % baryonic matter, 25.9 % Dark Matter and 69.2 % Dark Energy.
The XENONnT experiment is the current active detector of the XENON series of experiments, designed to detect WIMP (Weakly Interacting Massive Particles) Dark Matter. The detector is a so-called dual phase time projection chamber, utilizing the noble gas xenon as detection medium.
The XENON time projection chamber is a cylinder, mostly filled with liquid xenon and a gas layer on top. On the bottom and the top of the chamber arrays of light detectors, so-called photomultiplier tubes, are placed. Multiple electrodes throughout the time projection chamber allow for an electric field to be applied in the detector.
In case of an interaction of a particle, for instance a WIMP, energy is deposited inside the detector via elastic scattering with xenon atoms. The atom can now either be excited or ionized during this process. Relaxation of the excited states creates a scintillation light signal, called S1. Between the cathode (at the bottom of the time projection chamber) and the gate electrode (at the liquid-gas-interface) an electric field is applied, drifting the electrons from the ionization to the liquid surface. Between the gate and the anode (in the gas phase), a stronger electric extraction field is applied, dragging the electrons through the gas, where they excite further xenon atoms, creating a secondary scintillation light, called S2.
From the light pattern of the S2 signal in the photo multiplier tubes, and from the time difference between the S1 and S2 signal, the interaction site within the detector can be reconstructed with an accuracy in the order of millimeters. This allows for a strong background suppression, as photons and charged particles interact primarily in the outer regions of the detector, due to the excellent shielding properties of xenon. Dark Matter particles, however, can more easily reach the inner parts of the detector and can therefore be differentiated from background events.
XENONnT is located at the INFN Laboratori Nationali del Gran Sasso (LNGS) below the Gran Sasso massif in Italy. This places 1400 m of rock above the experiment, providing shielding from cosmic radiation, reducing the cosmic ray flux by a million. Additional background reduction is achieved by an active veto system in a 700 m³ water tank, surrounding the detector, identifying remaining cosmic muons and neutrons from spontaneous fission events close to the detector.
The XENON group from Mainz contributes to the experiment in various fields. These include data analysis tasks, as well as the development of novel, innovative detector technologies.
Mainz is involved in the Xenon Recovery and Storage (ReStoX) system, and the active veto system, which consists of the outer muon and the inner neutron detector, called muon veto and neutron veto. Operated as a Cherenkov detector, the muon veto consists of 84 photomultiplier tubes. Inside the muon veto, there is the neutron veto, surrounding the cryostat with the time projection chamber. The neutron veto has a volume of about 33 m³. This volume is optically separated from the muon veto by highly reflective expanded polytetrafluoroethylen panels, and it contains 120 PMTs.
It is designed to detect radiogenic neutrons from the detector materials that can mimic WIMP like signals inside the time projection chamber. Like the muon veto it detects the Cherenkov radiation of fast electrons which originate from Compton scattering of gamma-rays produced in the capture process of neutrons.
To enhance the neutron capture efficiency, the water has been doped with Gadoliniumsulfate-Octahydrate (GdSO) in 2024. To avoid the introduction of additional background, the raw materials are carefully chosen and the resulting GdSO is screened to verify that it has low intrinsic radioactivity. As this is an expensive and time-consuming process the GdSO will be recycled in case the water tank has to be emptied. To achieve this, Mainz is currently (summer 2025) providing a filtration facility, based on a nanofiltration process, to reduce the 700 m³ GdSO solution down to about 12 m³ – with minimal losses of GdSO. This system is developed together with the dutch company Lenntech Water Treatment Solutions.
First tested in XENON1T, and afterwards also successfully used in XENONnT, the Mainz group developed a novel calibration method utilizing 37Ar. This radioactive isotope decays at a very low energy of 2.82 keV, providing a calibration point close to the lower detection threshold of the detector. Due to the TRIGA reactor, located on the university campus, Mainz continues to produce and provide the calibration source to XENONnT for future calibration campaigns.
