Published in Eur. Phys. J. C: https://doi.org/10.1140/epjc/s10052-024-13472-4

Λ - Cold Dark Matter (Λ CDM) has been successful at explaining the large-scale structures in the universe but faces severe issues on smaller scales when compared to observations. Introducing self-interactions between dark matter particles claims to provide a solution to the small-scale issues in the Λ CDM simulations while being consistent with the observations at large scales. The existence of the energy region in which these self-interactions between dark matter particles come close to saturating the S-wave unitarity bound can result in the formation of dark matter bound states called darkonium. In this scenario, all the low energy scattering properties are determined by a single parameter, the inverse scattering length γ. In this work, we set bounds on γ by studying the impact of darkonium on the observations at direct detection experiments using data from CRESST-III and XENON1T. The exclusion limits on γ are then subsequently converted to exclusion limits on the self-interaction cross-section and compared with the constraints from astrophysics and N-body simulations.

The collaboration meeting took place in person at the Laboratori Nazionali del Gran Sasso from 3rd untill 5th July 2024. 

Published in PRD: https://doi.org/10.1103/PhysRevD.110.083038

The main goal of the CRESST-III experiment is the direct detection of dark matter particles via their scattering off target nuclei in cryogenic detectors. In this work we present the results of a Silicon-On-Sapphire (SOS) detector with a mass of 0.6 g and an energy threshold of (6.7 ± 0.2) eV with a baseline energy resolution of (1.0 ± 0.2) eV. This allowed for a calibration via the detection of single luminescence photons in the eV-range, which could be observed in CRESST for the first time. We present new exclusion limits on the spin-independent and spin-dependent dark matter-nucleon cross section that extend to dark matter particle masses of less than 100 MeV/c².

Published in Eur. Phys. J. C: https://doi.org/10.1140/epjc/s10052-024-13141-6

Cryogenic scintillating calorimeters are ultra-sensitive particle detectors for rare event searches, particularly for the search for dark matter and the measurement of neutrino properties. These detectors are made from scintillating target crystals generating two signals for each particle interaction. The phonon (heat) signal precisely measures the deposited energy independent of the type of interacting particle. The scintillation light signal yields particle discrimination on an event-by-event basis. This paper presents a likelihood framework modeling backgrounds and a potential dark matter signal in the two-dimensional plane spanned by phonon and scintillation light energies. We apply the framework to data from CaWO₄-based detectors operated in the CRESST dark matter search. For the first time, a single likelihood framework is used in CRESST to model the data and extract results on dark matter in one step by using a profile likelihood ratio test. Our framework simultaneously fits (neutron) calibration data and physics (background) data and allows combining data from multiple detectors. Although tailored to CaWO₄-targets and the CRESST experiment, the framework can easily be expanded to other materials and experiments using scintillating cryogenic calorimeters for dark matter search and neutrino physics.

After the completion of a three year long measurement period end of January, we quickly replaced the detector modules and performed some maintenance on the setup. The cool-down of the new detectors went smoothly and the data taking was restarted beginning of April. The focus in the upcoming measurement campaign lies once more on the study of the low energy excess. In particular we want to test if the excess events originate from the transition edge sensors themselves or their interface to the absorber crystals. Therefore most of the absorber crystals are equipped with two sensors. We are currently in the process of characterizing and calibrating the detectors and are eagerly awaiting the first exciting results.

Published at Comput. Softw.Big Sci.: https://link.springer.com/article/10.1007/s41781-024-00119-y

Cryogenic phonon detectors with transition-edge sensors achieve the best sensitivity to sub-GeV/c² dark matter interactions with nuclei in current direct detection experiments. In such devices, the temperature of the thermometer and the bias current in its readout circuit need careful optimization to achieve optimal detector performance. This task is not trivial and is typically done manually by an expert. In our work, we automated the procedure with reinforcement learning in two settings. First, we trained on a simulation of the response of three CRESST detectors used as a virtual reinforcement learning environment. Second, we trained live on the same detectors operated in the CRESST underground setup. In both cases, we were able to optimize a standard detector as fast and with comparable results as human experts. Our method enables the tuning of large-scale cryogenic detector setups with minimal manual interventions.

Published at J. of Low Temp. Phys.: https://link.springer.com/article/10.1007/s10909-024-03154-6

Recently low-mass dark matter direct searches have been hindered by a low energy background, drastically reducing the physics reach of the experiments. In the CRESST-III experiment, this signal is characterised by a significant increase of events below 200 eV. As the origin of this background is still unknown, it became necessary to develop new detector designs to reach a better understanding of the observations. Within the CRESST collaboration, three new different detector layouts have been developed and they are presented in this contribution.

Published in Eur. Phys. J. C: https://doi.org/10.1140/epjc/s10052-024-12647-3

Diamond operated as a cryogenic calorimeter is an excellent target for direct detection of low-mass dark matter candidates. Following the realization of the first low- threshold cryogenic detector that uses diamond as absorber for astroparticle physics applications, we now present the resulting exclusion limits on the elastic spin-independent interaction cross-section of dark matter with diamond. We measured two 0.175 g CVD (Chemical Vapor Deposition) diamond samples, each instrumented with a Transition Edge Sensor made of Tungsten (W-TES). Thanks to the energy threshold of just 16.8 eV of one of the two detectors, we set exclusion limits on the elastic spin-independent interaction of dark matter particles with carbon nuclei down to dark matter masses as low as 0.122 GeV/c². This work shows the scientific potential of cryogenic detectors made from diamond and lays the foundation for the use of this material as target for direct detection dark matter experiments.

Two contributions from CRESST collaboration were published at IDM2022:

"Latest observations on the low energy excess in CRESST-III"

 https://scipost.org/SciPostPhysProc.12.013

"Characterisation of low background CaWO₄ crystals for CRESST-III" 

https://scipost.org/SciPostPhysProc.12.031

The collaboration meeting took place in person at the Faculty of Mathematics, Physics and Informatics of the Comenius University campus in Bratislava from 12th untill 15th June 2023. 

 Published in Phys. Rev. D: https://doi.org/10.1103/PhysRevD.107.122003

We present limits on the spin-independent interaction cross section of dark matter particles with silicon nuclei, derived from data taken with a cryogenic calorimeter with 0.35 g target mass operated in the CRESST-III experiment. A baseline nuclear recoil energy resolution of (1.36 ± 0.05) eVnr, currently the lowest reported for macroscopic particle detectors, and a corresponding energy threshold of (10.0 ± 0.2) eVnr have been achieved, improving the sensitivity to light dark matter particles with masses below 160 MeV/c² by a factor of up to 20 compared to previous results. We characterize the observed low energy excess, and we exclude noise triggers and radioactive contaminations on the crystal surfaces as dominant contributions.