DarkSide-20k sensitivity, directional dark matter detection and the role of coherent elastic neutrino-nucleus scattering background

The existence of dark matter in the Universe is one of the most fascinating problems that modern physics needs to solve. Nowadays, the most intriguing candidate for dark matter is a so called Weakly Interactive Massive Particle (WIMP), that is a new, yet undiscovered, big-bang relic particle that interacts via gravity and possibly another kind of force. The golden method to observe dark matter is represented by direct searches that aim to spot effects of a possible interaction between a dark matter candidate with the target material of the detector. In this thesis, the 90% confidence level WIMP-nucleon sensitivity curve for DarkSide-20k, a dual-phase liquid argon time projection chamber of 20 tonne active mass to be built at LNGS, has been derived for the first time. This sensitivity curve depends crucially on the background content of the experiment. While DarkSide-20k will be able to keep the instrumental background content to less than 0.1 events for a total exposure of 100 tonne year, so called Coherent Elastic neutrino-Nucleus Scattering (CEnNS), that induces nuclear recoils almost indistinguishable from those potentially induced by WIMPs, will represent a major challenge. The detailed calculation of the expected number of CEnNS in the DarkSide-20k exposure and the study of the spectral distribution of those events has been carried out for the first time in this thesis, demonstrating that this background will be the dominant one for DarkSide-20k, despite the fact that it was neglected until this work. For even large exposures, the sensitivity will be strongly affected by CEnNS background. Thus, it is of utmost importance to find a way to distinguish a neutrino from a WIMP interaction. In this thesis, the potentiality of a directional tonne-scale DM detector located at LNGS has been studied in detail. The results obtained confirm that the development of experimental technologies able to couple directional sensitivity with large fiducial masses (many tonnes) and the ability to collect large exposures free of background from beta/gamma events and neutron-induced nuclear recoils is a priority for future dark matter detectors. Finally, the uncertainty on the CEnNS scattering cross section affects significantly the number of expected background events and it is dominated by the nuclear form factor parametrization. Since neutrinos couples preferentially with neutrons, the lack of knowledge of the neutron distribution and its associated radius plays a fundamental role. In this contest, the first experimental observation made in 2017 by the COHERENT Collaboration of a CEnNS process, provides a valuable occasion to experimentally constrain for the first time the CEnNS phenomenology. In this thesis, the first determination of the average neutron radius of Caesium and Iodine and the difference between the neutron and proton radii, known as the "neutron skin", has been obtained. The radius of the neutron density distribution is a crucial ingredient for understanding the background of future dark matter detectors and also the nuclear matter equation of state, which plays an essential role in understanding several processes, like nuclei in laboratory experiments, heavy ion collisions, the structure and evolution of compact astrophysical objects as neutron stars.