Searching for Dark Matter and New Physics in the Neutrino sector with Cryogenic detectors

A brief introduction to the dark matter puzzle and the field of direct detection, with a particular emphasis on the neutrino background issue and the Edelweiss experiment, is given in Chapter 1. An above-ground dark matter search, demonstrating the low-mass reach achievable with the Edelweiss technology, is presented in Chapter 2. This result also demonstrates the feasibility to achieve sub-100 eV energy threshold on 30 g-scale Ge crystals operated from the surface, as planned for the forthcoming Ricochet experiment. Chapter 3 gives a detailed overview of the future Ricochet low-energy neutrino observatory, hosting two cryogenic detector arrays: CryoCube and QArray, that will be deployed at ILL in the coming years. A preliminary experimental setup, as well as background estimates and new physics reaches will be presented. Additionally, we also give an extensive discussion about various systematics that may affect our measurement, and how we plan on mitigating them. Eventually, we present our onsite characterizations, including fast neutron and vibration measurements which are both major threats to Ricochet's CENNS sensitivity. Chapter 4 describes the CryoCube detector design consisting in an array of 27 Ge crystals of about 30 g heat sunk at 10 mK and readout with dedicated HEMT-based preamplifiers. Firstly, the chapter reviews the electrothermal modeling and optimization of the heat channel, then it discusses the electrostatic modelling and optimization of the ionization channel. Lastly, the chapter assesses the CryoCube sensitivity to CENNS with the Ricochet experiment at ILL, and demonstrates that discovery significance ranging from 17-to-5, depending on the various background and setup assumptions, should be achievable after only one reactor cycle. Finally, this manuscript ends with Chap. 5 which summarizes the various individual 30 g-scale Ge detector performance, how they compare with the modeling from the previous chapter, and how we plan to further improve them in order to achieve the CryoCube targeted performance. (author)