Dottorato in FISICA

Titolo della tesi: Search for New Physics with CUORE and CUPID

Frontier research in physics has a manifold contribution to science: while it tries to unveil the mysteries of the Universe it develops cutting edge technologies. For instance, the hunt for rare processes and feebly interacting particles, led to the realization of the coldest volumes in the Universe. These are meant to operate cryogenic calorimeters, a technology capable to detect the heat released by a single particle crossing a material. The elements composing cryogenic calorimeters are crystals instrumented with very sensitive temperature readers, able to convert a small heat release into an electric pulse, and a cryogenic facility to keep them stably at low temperature, of the order of 10 mK. Indeed, thanks to the low operation temperature, these detectors turn the energy deposited by particle interaction into a measurable temperature increase, typically as small as a few micro Kelvin. This mechanism is the key to achieve an excellent resolution, typically well below 1% at a few MeV deposited energy. But the most appealing feature of this technology is versatility in the choice of the crystal. As far as it ensure a low heat capacity, any crystal can be operated as cryogenic calorimeter. This allows to choose crystals containing the isotope most suitable for a specific research field. Beside being among the coldest place of the Universe, facilities for cryogenic calorimeters are likely among the most quiet. Indeed, all their components have been studied and optimized over the years to create extremely low background environments. In particular, the experimental volume hosts only radiopure materials and it is shielded against natural radioactivity and cosmic rays. To have an idea, the detectors I describe in this thesis typically detect less than an event per day in a kg of material. In this way, they are sensitive to phenomena less likely to happen than once in the Universe lifetime. The efforts to develop cryogenic calorimeters are meant to provide an answer to the most compelling open questions about our Universe. The hunt for the Majorana neutrino, for example, aims at explaining why we are made of only matter, while we don't observe antimatter anywhere. To solve this large scale asymmetry, we should find some kind of interaction that creates more matter than antimatter, that eventually happened in the early Universe. A Majorana particle coincides with its own antiparticle, therefore it could give rise to such processes. Among the particles we observe in nature, neutrinos are the only candidate to be of the Majorana type. This hypothesis leads to the existence of the neutrinoless double beta decay, the most appealing process to observe lepton number violation, and eventually to provide a key ingredient for the matter antimatter asymmetry issue. Certainly, one of the most fascinating mystery of the Universe is the existence of dark matter. It was inferred by anomalous gravitational effects that can be explained by the presence of halos of particles enveloping the galaxies. However, none of the observed particles behave as dark matter do, since it does not show any electromagnetic interaction. Moreover, the evolution of the Universe needs the dark matter to be made by cold, slow moving, particles, so that it can explain the formation of galaxies and larger scale structures that we observe today. This evidence, for example, excludes neutrinos as dark matter candidates because, despite being neutral, they are too light and would be too fast in the early Universe. An appealing solution to dark matter are the axions, light bosons able to populate the early Universe and behave as ordinary matter from the gravitational point of view. Despite being very light, their bosonic nature, on contrary to fermions like neutrinos, allows them to behave coherently as a macroscopic object. Therefore, they can account for the lack of matter during the Universe expansion and explain astrophysical scale dark matter phenomena. Furthermore, axions were not originally proposed for dark matter, but to solve a controversy related to the neutron electric dipole moment, which is predicted to be non-zero by the Standard Model of particle physics, against what observed experimentally. So, the growing appeal for this particle, is well justified by the fact that it would solve two puzzles of modern physics at once. The Sun, thanks to the high temperature and high density of its core, together with its proximity to Earth is a good candidate to produce a flux of axions, to be detected on Earth through its feeble coupling to ordinary particles. The hypothesis of axions paved the way to several analogous models predicting particles with similar properties, often called axion-like particle, or ALPs. One of these is the majoron, which is coupled to the Majorana neutrino, and therefore it would be emitted in the final state of neutrinoless double beta decay. The features offered by cryogenic calorimeters well fit this kind of searches. Indeed, a precise energy reconstruction, very low background and a large exposure, significantly contribute to improve the sensitivity to events induced by rare decays such as the neutrinoless double beta decay as well as by feebly interacting particles such as the axions. My PhD activity was done in the framework of CUORE (Cryogenic Underground Observatory for Rare Events), CUPID (CUORE Upgrade with Particle IDentification) and the demonstrator CUPID-0, cryogenic calorimeters experiments built or projected to search for neutrinoless double beta$ decay. However, the reliability of their technology opens the door to perform a broader spectrum of studies. In particular, the analysis I carried on during these years were focused on the search for majorons and solar axions. While the signature of the neutrinoless double beta decay consists in a monochromatic peak at a few MeV, the emission of a majoron in the final state modifies it, giving rise to a continuous energy spectrum. Therefore, the search for this particle needs a spectral analysis to understand how its shape contributes to the background. Solar axions instead, may give rise to different experimental signature depending on how they are produced and detected, Among these, the most simple is a monochromatic peak laying about 2 orders of magnitude below the neutrinoless double beta decay one. In principle, the technology operated by these experiments allows for a wide energy range of operation, and for a sensitivity across about three orders of magnitude. Nonetheless, the lower the energy, the worse the average detector performance, therefore a dedicated analysis is needed to refine the energy spectrum. The three experiments I've been working on are closely interconnected. CUORE started its data taking in 2017, with the purpose of operating the first ton-scale cryogenic calorimetric experiment. After about 2 years of cryogenic improvements and maintenance, it reached a full time operation still ongoing. The CUPID-0 demonstrator set the basis for the CUORE upgrade, by adding the particle identification through the simultaneous read out of the calorimetric signal and the scintillation light. It started to acquire data in 2018 and it was decommissioned in 2020, with the purpose of using the cryogenic facility for the development of the future experiment CUPID. The latter will join the efforts of CUORE, CUPID-0 and the demonstrator CUPID-Mo to be a ton scale cryogenic calorimeter experiment operating the heat-light simultaneous read out. The beginning of my PhD coincided with a fruitful period for the cryogenic calorimeters development. Several measurements were, and are at the time of writing as well, ongoing to optimize the performance of the future CUPID detector, and to understand its weaknesses. In 2021, I joined a prototype measurement at LNGS meant to test different mechanical configuration to improve the light collection, and eventually the particle identification capabilities. Moreover, in 2021 CUPID-0 had just ended its data taking, and I had the opportunity to use a set of fully processed data to perform a Bayesian analysis to study an exotic process such as the emission of a majoron in neutrino less double beta decay. Finally, I joined the CUORE collaboration and the low energy working group with the aim of searching for solar axions events. The processing of a few to a few tens of keV energy deposit events in CUORE is not trivial due to its noise threshold and its sensitivity to the vibrational environment. I developed a strict data quality selection and analysis methods, with the purpose of studying the CUORE energy spectrum down to about 10 keV and search for the solar axion peak at 14.4 keV. In this work I present the results from the 2nd half of the CUORE data taking, while I foresee to implement the analysis of the full exposure and finalize the results in the upcoming months. In this thesis I enter the details of the techniques behind cryogenic calorimeters and I describe their application as detectors devoted to neutrino less double beta decay search (Chapter 1). Then, in Chapter 2, I give an overview of the theoretical framework predicting axions and axion-like particles, including majorons. In Chapter 3, I describe the CUORE detector. In the same Chapter, I outline the data processing and analysis techniques of CUORE, most of which are common to other cryogenic calorimeter experiments such as CUPID-0 and CUPID. In Chapter 4, I explain the techniques leading from low energy deposits in the detector to a large exposure energy spectrum in CUORE. Here, I outline the methods I developed to clean the spectrum from non-physical events, and improve the overall data quality in this energy region. The following Chapter (5) is devoted to the study of the background shape in the region of interest to search for the solar axion peak. Chapter 6, describes the CUPID-0 detector, the techniques to reconstruct the background contributions and the study I performed to search for majoron emission in neutrino less double beta decay. Finally, in Chapter 7, I present the results we achieved with the research and development measurement at LNGS of the first single module of CUPID, aimed at optimizing the mechanical structure and the light collection.