Simulation for the search of dark matter with CRESST

Over 80% of all matter in the universe is not visible, the so-called dark matter. This dark matter could be explained by introducing a new particle, which interacts weakly only with ordinary matter. However, to observe it, the interaction has to be stronger than the gravitational one. The predictions for the mass of this weakly interacting particle spans over several orders of magnitude. Recently, models with a mass range for dark matter particles between a few MeV and a few GeV have generated much interest. The CRESST experiment, which is currently being carried out at the Gran Sasso laboratory in Italy, is optimized for dark matter by searching for dark matter particles scattering with conventional matter. The energy of the scattering process is converted into lattice excitations of the detector crystal, the so-called phonons, and then read out with a sensor at the transition temperature. With the data collected so far by CRESST, no signal has been observed, however, the exclusion limits for dark matter candidates with a mass of less than 2 GeV are the best limit among the experiments for the direct search for dark matter. In the first funding period, the detection threshold for the recoil energy could be lowered down to 30 eV, an improvement of an order of magnitude compared to previous CRESST detectors and one of the lowest thresholds of all experiments. At recoil energies below 200 eV, however, the event rate rises sharply, and the origin of these energy deposits is not yet understood. Studies to identify these energy deposits are a central part of the project. These studies are based on detailed simulations studies and the analysis of experimental data. In addition to decoding this unknown background, different detector materials for the crystals, e.g., sapphire, are to be used in addition to the previously used detector material, calcium tungstate. These crystals and their properties must also be simulated to make full use of the data.

The mystery of dark matter is one of the great unsolved questions of modern physics. One possible solution is the existence of massive particles that do not interact electromagnetically and thus elude most detection methods. The CRESST experiment, which is being conducted at the Gran Sasso underground laboratory in Italy, is searching for these particles using cryogenic detectors and quantum sensors. Due to the low interaction of dark matter with the detector, background processes from rare radioactive decays can mimic signal events and thus influence the result. It is therefore important to estimate the number of background events as accurately as possible. As part of the project, several methods were developed using Geant4 simulation software to estimate background processes due to radioactive decays for the CRESST experiment. Two methods were used to estimate the contributions from radioactive decays. The first method used information from radioactive decay chains, which allows measurements of alpha decays at higher energies to be linked to beta decays in the signal region. In the second method, the simulated decay spectra of various possible radioactive decays were fitted to the entire measured decay spectrum. The information from the decay chains was used as input parameters. In addition to the active cryogenic detectors, the experimental setup in which they are operated was also simulated and possible external background contributions were estimated. A database was created for the results of different detector materials and geometries. In addition to the direct measurements, information from external precision measurements of the radioactivity of the passive detector material was also collected and used for the simulation. The results were compiled in a database. The studies revealed that the nature of the crystal surface influences the contribution of radioactive background processes. Various detector surfaces were simulated and the influence on the measurement was quantified. The search for dark matter with CRESST did not find any evidence of its existence. In order to set a limit on the sensitivity of the interaction rate and mass of hypothetical dark matter particles, an adjustment was made to an astrophysical model, taking into account the estimation of radioactive background processes.