F. Wagner, Towards next-generation cryogenic dark matter searches with superconducting thermometers, Ph.D. thesis, Technical University of Vienna
Today we observe overwhelming gravitational evidence for the existence of dark matter in the universe and its non-null abundance along the solar circle. Experiments using cryogenic calorimeters spearhead the effort to measure the scattering of sub-GeV/c2 dark matter particles with nuclei of a detector target. Among them, the CRESST experiment achieves the strongest sensitivity for spin-independent, elastic scattering scenarios. Its spin-off, the COSINUS experiment, is set up to validate the long-standing dark matter discovery claim of the DAMA experiment, exploiting low energy thresholds and particle identification. These searches rely on careful detector design, measurement setup, and data analysis to provide insights into the nature of dark matter. We review the motivations for the searches, the operation principles, and study detector design choices in detail by constructing a dedicated detector response simulation. The analysis techniques are summarized and implemented in a modern software toolbox, allowing the execution of all established methods and the incorporation of machine learning classifiers. Building upon these methods, we summarize the characterization of two detector modules from the latest CRESST-III measurement campaign, which lead to the currently most stringent limits on spin-dependent sub-GeV/c2 dark matter-nucleus elastic scattering. Furthermore, we study detector prototypes for COSINUS and estimate achievable energy thresholds of detector designs for the first physics runs. We describe the technological challenges for a large-scale measurement setup and analysis process and study methods based on deep and reinforcement learning with CRESST-III data to automate the required manual interventions. These methods equip the next generation of cryogenic dark matter searches with improved sensitivities and higher collected exposure.
K. Shera, Studies on the remoTES-based cryogenic calorimeters for the COSINUS experiment, M.Sc. thesis, Technical University of Munich
The existence of dark matter has been supported by a significant amount of evidence, yet its nature remains an open question. DAMA/LIBRA experiment claims to observe a signal that can be interpreted as evidence for dark matter in our galaxy. The Cryogenic Observatory for SIgnals seen in Next generation Underground Searches (COSINUS) experiment, located at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy, aims to provide a model-independent cross-check of this observed signal by utilizing the same target material (NaI). COSINUS operates NaI at very low temperature of O(mK). The phonon signal resulting from a particle interaction in the NaI crystal is measured by a Transition-Edge Sensor (TES). TESs are thermometers made of superconducting thin films usually deposited onto the absorber surface. The deposition of TES on NaI is particularly challenging due to the hygroscopic nature and low melting point of this crystal. To overcome this limitation, COSINUS was the first experiment to implement the remoTES readout design to measure the phonon signal; a proof-of-principle was successfully accomplished. The novelty of the remoTES design consists in depositing the TES on a separate wafer and remotely connecting it to the absorber via a gold link. This approach ensures that the crystal is not subjected to the fabrication process of the TES. The remoTES readout design is a flexible and easy-to-fabricate alternative that could be beneficial for other rare-event searches, enabling mass fabrication and better reproducibility among detectors. The optimization of the remoTES readout design is crucial to meet the sensitivity goal of the COSINUS phonon detector threshold of 1 keV (200 eV baseline resolution). The primary objective of this thesis is to systematically study and understand the impact of different detector components on the overall detector performance with the aim to achieve the best
possible sensitivity. Since handling NaI is challenging the studies were performed on silicon (Si) which is a well-studied material. They involved varying the gold link properties, which includes the gold pad thickness and bonding technique, and TES geometry. It was observed that the above-mentioned parameters influence the performance of the detector prototypes. By optimizing the design a baseline resolution of 89 eV was reached.
A. Fuss, Simulation based Neutron Background Studies for the CRESST and COSINUS Dark Matter Search Experiments, Ph.D. thesis, Technical University of Vienna
The true nature of dark matter is one of the key puzzles in modern physics. While large scientific consensus has been achieved about its existence, in contrast to theories about modified gravity, an experimental proof still has to be found. Various experiments are employing complementary techniques to probe different masses and interaction mechanisms of potential dark matter particles. With so-called direct searches, dark matter in our local galactic halo shall be probed. CRESST has lead the path for direct detection searches over many years, pioneering the development of cryogenic detectors, especially suited for detection of low-mass dark matter. A simultaneous phonon and scintillation light read-out is used for particle detection and discrimination. Looking for dark matter-induced nuclear recoils, this method leads to great reduction of the background due to electrons and gammas. Employing the same detection technique but different target material (NaI instead of CaWO4), the COSINUS experiment is aiming to cross-check the results of the DAMA/LIBRA experiment. DAMA/LIBRA is reporting an annually modulated signal interpreted by its proponents as an actual model-independent evidence of galactic
dark matter. However, this interpretation is not generally accepted by the community. COSINUS will thus use the same target material as DAMA/LIBRA, i.e. NaI, and is currently in prototyping and early construction phase.
In both CRESST and COSINUS, neutrons entering the detectors can lead to nuclear recoils indistinguishable from a dark matter-induced signal and are thus considered a dangerous background. To shield against neutrons and other types of background, the experimental setups are hence located in deep underground laboratories and use
dedicated shielding layers surrounding their detectors. This work presents neutron background simulation studies performed for CRESST and COSINUS to estimate the spectra, rates and sources of detectable neutrons. For this purpose, the detailed geometry of the corresponding setup was implemented in a Geant4-based particle simulation software, together with many new features necessary to perform the desired simulations of neutrons originating from muon interactions, (𝛼,n) reactions and spontaneous fission processes. To compare to measured data, a postprocessing tool was greatly extended, considering the time and energy resolution of the detectors.
Massimo Girola, Characterization of Scintillating Cryogenic Calorimeters, M.Sc. thesis, Milano - Bicocca University
…Across the various experimental approaches used to test the long-standing DAMA/LIBRA claim on the evi- dence of DM, COSINUS has the primary goal of providing a model-independent cross-check using the same target material (NaI) operated as a scintillating cryogenic calorimeter. In the framework of the COSINUS experiment, I have characterized the response of a NaI crystal with an NTD read- out and a Ge-wafer LD operated at 12 mK in a cryostat placed underground at LNGS. The setup has shown good overall performance. In particular, through an energy calibration based on the only available peak (59.54 keV γ from 241Am), I have estimated an energy threshold of about 0.5 keV, a value that overcomes the ultimate goal set by the COSINUS collaboration, which is ≃ 1 keV and is better than the one obtained with other thermal sensors for the NaI readout, which usually reach ∼ 10 keV. I have also measured a discrimination power between β/γ and neutron-induced nuclear recoils of ≃ 4.68, high enough for particle discrimination purposes. Thanks to this setup, it was also possible to give a rough estimate of the quenching factor (in the light channel) of Na recoils with respect to β/γ rays, which resulted to be in agreement with the value present in the literature…
V. Zema, Unveiling the nature of Dark Matter with Direct Detection Experiments, Ph.D. thesis, Gran Sasso Science Institute
…About one-third of this dark sector is associated to an invisible and still undetected form of matter, the so-called dark matter, whose gravitational effect manifests at all cosmological scales. Both theoretical and experimental observations based on ordinary gravity reinforced the evidences for the existence of DM, since its first appearance in the pioneering calculations of F. Zwicky (1933). This PhD project explores the hypothesis that DM is made of new particles beyond the standard model. More specifically, it focuses on those DM particles which are trapped into the galactic gravitational field and populate the galactic halo. If DM interacts with ordinary particles, extremely sensitive detectors operating in very low-background environments, are expected to detect galactic DM particles scattering off their target material. This widely employed experimental technique is known as DM direct detection and it is the focus of my studies, where I consider the further hypothesis that DM interacts with atomic nuclei. The research I conducted during my PhD program consists of two main parts: the first part focused on purely phenomenology aspects of the DM direct detection (namely on the DM annual modulation treated using a non-relativistic effective theory and on the scattering of spin-1 DM particles off polarised nuclei) and the second one is more closely connected to experimental applications. The latter has been strongly stimulated by my collaboration with the two DM direct detection experiments CRESST and COSINUS. For CRESST, I compute the DM-nucleus cross-section for the conventional spin-dependent interactions, used to analyse the data collected with a prototype Li-based detector module, and I derive some prospects for a time dependent analysis of CRESST-III data, using a statistical frequentist approach based on Monte Carlo simulations. For COSINUS, I provide a significant extension of the pulse shape model currently used by CRESST and COSINUS in order to explain experimental observations related to the COSINUS detector response. Finally, I contribute to ongoing studies on the phonon propagation in NaI crystals based on solid state physics. This PhD thesis has been oriented to fill the gap between theoretical and experimental efforts in the DM field. This approach has facilitated the exchange of expertise, has driven the trend of my research and has stimulated the development of the ideas and methods described in this PhD thesis.