ELOISE: Reliable background simulation at sub-keV energies

Physics strives to describe our world in ever-increasing details and completeness by searching for new phenomena the so-called New Physics. One approach is to measure known phenomena with high- precision. Any deviation of this measurement from our knowledge of know physics may indicate New Physics. For this purpose, the NUCLEUS experiment aims to measure the coherent elastic neutrino nucleus scattering. Here, the elementary particle neutrino scatters off a whole atomic nucleus (hence coherent) while energy and momentum are conserved (hence elastic). In case of NUCLEUS, the atomic nucleus is bound in crystals of sapphire or calcium tungstate. During the scattering, the nucleus may gain sufficient energy to leave its place in the crystal. Afterwards the nucleus may transfer its energy to other nuclei in the crystal. To be able to search for New Physics with NUCLEUS, we have to understand precisely the scattering and the subsequent energy transfer. Only with this knowledge, NUCLEUS has a chance to fulfil its objective and may contribute to an improved understanding of our world. In principle, we know how the scattering and the energy transfer works in crystals. However, it was never attempt to use this basic knowledge to derive a high-precision description for sapphire and calcium tungstate. This is especially true for the energy transfer. We, the scientist of the ELOISE project, try to change this. Starting from the basic principles, we will describe the scattering and energy transfer in a physical model. Based on this model, we will develop a computer program to predict actual numbers for the energy transfer to other nuclei in the crystal. We will compare these predictions with measurements of the energy transfer. For this, we will use already existing measurements published by other scientists, but we will also do dedicated measurements ourselves. Because we start from basic principle, we expect that the first comparison of prediction and measurement will reveal discrepancies. By analysing these discrepancies, we can gain new insights and can improve the model of energy transfer. The computer program will then use the improved model to make new predictions, which will be again compared to measurements. The cycle of improvement, prediction and comparison will be repeated several times to increase the precision of the model. Our results will be not only necessary for NUCLEUS and its search for New Physics. Each application that relies on a precise understanding of the energy transfer in sapphire or calcium tungstate will benefit from our insight.