Dottorato in FISICA

Titolo della tesi: Search for low-mass dark matter with direct detection experiments

The present thesis, in the context of the direct detection of dark matter, focuses on the search for low mass dark matter candidates. We set the most stringent bound on the dark matter spin independent cross section in the GeV/c^2 mass range using the DarkSide-50 experiment. We extend the bound of the experiment in the sub-GeV region thanks to the exploitation of the Migdal effect. Finally, we investigate the possibility of measuring such an effect with the CYGNO experimental approach. At present, dark matter is only known via gravitational effects and its nature, e.g. its interactions with ordinary matter or its mass, has not yet been discovered. The search for dark matter weakly interacting massive particles with noble liquids has probed masses down and below a GeV/c^2. Detecting the scattering of dark matter particles in the sub-GeV ``low mass'' range is a challenging task, since, in this mass region, the typical energy transfer is below the experimental threshold. However, the nuclear recoils induced by dark matter particles interacting in the detector can be followed by the excitation and ionization of the recoiling atom, the Migdal effect. We improve the preliminary results of the 2018 DarkSide-50 analysis thanks to a refined detector response model, data selection, and background model. We develop an innovative analysis tool in the Bayesian approach that is able to reproduce the detector response in a semi analytical way. As a result of the analysis, the DarkSide-50 observed sensitivity turns out to be the world's best sensitivity in the m_{DM} = [0.7,4.7] GeV/c^2 mass region, being able to exclude at 90\% credible interval a dark matter spin independent (SI) cross-section sigma_{DM}^{SI} = 2.1 x 10^{-43} cm^2 at m_{DM} = 4.5 GeV/c^2. We compute the rate of the so-called Migdal effect for hydrogen, helium and for argon atoms: thanks to the inclusion of this effect, the sensitivity of DarkSide-50 can be extended to lower masses, down to 60 MeV/c^2, where the experiment has currently the strongest sensitivity. Up to this day, the Migdal effect has been measured only in alpha and beta decays, and its verification in nuclear scattering would be a strong confirmation of the results obtained by the direct detection experiments. For this purpose, we investigate the possibility of measuring the Migdal effect induced by a neutron source with a prototype of the CYGNO experiment. With its light target atoms and great 3D position resolution, CYGNO will be a complementary experiment with respect to DarkSide, being able to access the direction of the nuclear scatterings and potentially distinguish the dark matter induced recoils from the irreducible neutrino coherent scattering background.