The role of marine microseisms in shaping the performance of CUORE and advancements for CUPID experiment

Neutrinos are among the most enigmatic elementary particles in the Standard Model, with several key properties still unknown. The observation of the neutrino flavour oscillation in 1998 led to the discovery that neutrinos are massive particles. However, the absolute values of their masses are yet to be measured. The ordering of the three neutrino mass eigenstates is also unknown. Moreover, unique among all the elemen- tary fermions in the Standard Model, neutrinos might be Majorana fermions, therefore coinciding with their own antiparticles. The hypothesis of Majorana neutrinos can be experimentally probed by searching for neutrinoless double beta (0νββ) decay. 0νββ decay is a beyond Standard Model process, where a nucleus decays by emitting two elec- trons/positrons without emitting the corresponding antineutrinos/neutrinos, therefore violating the conservation of the lepton number. The discovery of 0νββ decay would assess the nature of neutrinos as Majorana fermions, provide the first evidence of lepton number violation and of leptogenesis, and provide insights on the absolute scale of the neutrino masses and of their ordering. A world-wide scientific effort is ongoing in order to search for 0νββdecay, exploiting various experimental techniques. CUORE, operating at Laboratori Nazionali del Gran Sasso (LNGS) in Italy, is among the leading experiments in the field, searching for 0νββ decay of 130Te by deploying a ton-scale array of low-temperature calorimeters operated at the mK-scale. Inheriting the legacy of CUORE, the search for 0νββ decay with low-temperature calorimeters will be carried forward by the next-generation experiment CUPID. The research activities described in this document have been carried on in the framework of the CUORE and CUPID experiments. In chapter 1 we will provide a general overview of the main properties of neutrinos and of double beta decay. We will also report the main experimental challenges in searching for 0νββ decay and the most updated limits set by various experiments. In chapter 2 we will provide a general overview of low-temperature calorimeters and of Neutron Transmutation Doped (NTD) thermistors, the class of temperature sensors implemented in CUORE and foreseen to be exploited also in CUPID. Finally, we will 1 Introduction 2 present a general overview of the working principle of 3He-4He dilution cryostats, as the technology required to reach the mK-scale and to stably operate low-temperature calorimeters at such temperatures. In chapter 3 we will provide a general overview of the CUORE experiment. We will describe the detectors array and the experiment structure, the various techniques imple- mented in order to suppress the noise affecting CUORE low-temperature calorimeters, and the workflow of data acquisition and processing. Finally, we will report the latest experimental limits on the search for 0νββ decay of 130Te. In chapter 4 we will first provide a brief overview of the CUPID next-generation experiment. We will then focus on the design, operation and data analysis of the first CUPID full-tower prototype. The operation of such detectors tower was a milestone achievement for the CUPID project, being the first test of the detectors technology on a full-scale prototype. I contributed to this collaborative project during both the installation phase of the prototype tower within its hosting cryostat at LNGS, and during the phases of detectors operation, data acquisition, and analysis. In chapters 5, 6 and 7, we will present a novel multi-device analysis based on the cross-correlation between Earth observation data, seismometric data and CUORE data, developed with the aim of assessing and characterizing the interplay between a class of environmental phoenomena, namely marine microseisms, and the performance of CUORE. Indeed, we assessed for the first time ever the detection of marine microseismic vibrations through low-temperature calorimeters operated at the mK-scale, identifying microseismic vibrations as a novel, previously unresolved source of noise in the sub-Hz domain for detectors operated at the mK-scale. In chapter 5 we will first provide a brief overview of the environmental sources of marine microseisms. We will then introduce Copernicus, the European Union space pro- gramme dedicated to Earth observation. We exploited Copernicus data in order to assess the time evolution of the Mediterranean Sea activity, a key parameter characterizing the sources of marine microseisms. Finally, we will assess the correlation between marine data from Copernicus and seismic data from seismometers installed at LNGS, thus pro- viding the foundation to directly correlate Copernicus data with data from CUORE low-temperature calorimeters. In chapter 6 we will describe the analysis technique developed to correlate Coperni- cus marine data during severe transient marine events and the performance of CUORE low-temperature calorimeters. Indeed, we assessed that the outbreak of storms in the Mediterranean Sea reflects into an increased vibrational sub-Hz noise affecting CUORE low-temperature calorimeters. Moreover, we will discuss the response of the experimen- tal infrastructure of CUORE to the external action of marine microseismic vibrations. In chapter 7 we will extend the correlation analysis between Copernicus and CUORE data, covering a time scale of 4 yr, in order to assess if the seasonal modulation of the Introduction 3 Mediterranean Sea activity reflects into a seasonal modulation of some key parameters of CUORE. Indeed, we assessed that the baseline resolution, the energy resolution at γ peaks, and the low-energy threshold and exposure, all exhibit a seasonal modulation driven by the variation of the marine activity. We will also quantify the impact of ma- rine microseismic noise on the experimental sensitivity of CUORE to the search of 0νββ decay of 130Te. Finally, in chapter 8 we will describe an experimental campaign performed in the framework of the upgrade of the CUORE cryostat in sight of both the second phase of the CUORE scientific program and of CUPID. The cryogenic upgrade foresees the replacement of the pulse tubes (PT) cryocoolers installed in the CUORE cryostat, as well as the improvement of their thermal coupling and mechanical decoupling from the cryostat itself. To achieve this goal, we foresee to replace the OFHC-Cu PTs thermal- izations currently installed in CUORE with high-purity aluminum thermalizations. At low temperatures ( 40 K) high-purity aluminum is a better thermal conductor with respect to OFHC-Cu, while also being a softer and more flexible material. Aluminum thermalization will therefore be more effective in damping the propagation of vibrations from the PTs to the detectors. In chapter 8 we will describe the experimental cam- paign of comparative measurements of the thermal conductivity of a sample of copper, implemented for the realization of the PTs thermalization in CUORE, and various sam- ples of high-purity aluminum, characterized by different purity levels and by different geometries.