The CUORE experiment: detector optimization and modelling and CPT conservation limit

My PhD thesis project has been carried out within the CUORE (Cryogenic Underground Observatory for Rare Events) experiment. The CUORE experiment is a ton scale cryogenic detector installed in the underground Gran Sasso laboratories of INFN (LNGS) in Italy. The CUORE detector consists of an array of 988 TeO2 bolometric detectors (5 x 5 x 5 cm3 each) arranged into 19 identical structures called 'towers'. Each tower hosts 52 bolometers arranged in 13 floors, each containing 4 crystals. Each crystal has a mass of 750 g, resulting in a total mass of 742 kg of TeO2, or 206 kg of 130Te. The goal of the CUORE experiment is to investigate the 0nuBB decay of 130Te. My main activities have been related to the CUORE detector operation, characterization and optimization. Moreover I developed software tools for the CUORE data processing and analysis. In addition, I have been investigating the CUORE sensitivity to rare processes other than the 0nuBB decay, like the CPT symmetry violation in the 2nuBB decay. In the first part of my PhD thesis, I give a detailed review of the CUORE detector optimization campaigns performed in 2017. It was the first time that such a large number of bolometric detectors were simultaneously operated in a completely new and unique cryogenic system. My main contribution has been related to the analysis of the detectors performance in terms of signal optimization and noise reduction, in order to set both the optimal operating temperature of the cryogenic system and the polarization voltages (working points) for the NTDs coupled with each bolometer. In order to set the best operating conditions of the CUORE detectors, it has been necessary to characterize the response of the CUORE bolometers and NTDs. Utilizing the high number of detectors in CUORE, it was possible to study and model more features of the bolometers response and to try to develop a complete (static and dynamic) thermal model. The development of a thermal model can contribute to the identification of the physical parameters which are affecting the CUORE bolometers energy resolution and which could be possibly better optimized. The second part of the work is focused on the analysis of the CUORE data and on the study of the potential of the CUORE experiment for the search for rare events and/or for physics beyond the Standard Model other than the 0nuBB decay of 130Te. The first CUORE physics data were acquired during the summer of 2017, with a total collected 0nuBB exposure of 86.3 kg.yr of (nat)TeO2. The analysis of these data lead to the first CUORE 0nuBB half-life limit and to a more precise measurement of the 2nuBB decay half-life for 130Te. I have contributed to the analysis by developing and debugging the software for processing and analyzing the CUORE data; moreover I have worked to the evaluation of the selection efficiencies which have been used in the production of the CUORE physics results. Furthermore I studied the sensitivity of the CUORE experiment to the possible CPT symmetry violating terms. Indeed, the violation of this symmetry is described in several Standard Model Extensions (SME); in the case of isotopes which can undergo double-beta decay, the CPT violation would induce a deformation in the spectrum of the total energy of the two electrons emitted in the 2nuBB process. Utilizing the data acquired by CUORE in 2017, it was possible to set the first limit for the CPT violation evaluated from the 130Te isotope.