DARK MATTER DIRECT DETECTION WITH THE DARKSIDE PROJECT RED: AN EXPERIMENT TO PROBE THE RECOIL DIRECTIONALITY IN LIQUID ARGON

Abstract

The existence of dark matter in the Universe is nowadays commonly ac cepted as the explanation of many astrophysical and cosmological phenom ena, ranging from internal motions of galaxies to the large scale inhomo geneities in the cosmic microwave background radiation and the dynamics of colliding galaxy clusters. Cosmological and astronomical observations, supported by the recent results from the Planck satellite, indicate that dark matter, which forms the observed large-scale structures and galaxies, ac counts for 27%, dark energy, responsible of the observed accelerated expan sion of Universe, accounts for 68% while the remaining 5% is composed by ordinary baryonic matter. Elementary particle physics o↵ers an attractive solution to explain non baryonic dark matter in the form of relic Weakly Interacting Massive Parti cles (WIMPs), formed in the early Universe and gravitationally clustered to gether with the standard baryonic matter. In our galaxy, dark matter might constitute a halo, extending far beyond the visible disk, whose properties are inferred from the rotational kinematics of the visible matter. WIMPs could then be directly detected, as the Earth passes through such a halo, by looking at the nuclear recoils produced by WIMP interactions with ordi nary matter. Up to now none of the running, neither the already concluded, experiments were able to detect dark matter as a particle. In this scenario, dual-phase noble liquid Time Projection Chambers (TPC), detecting both ionization and scintillation lights produced by recoiling nu clei, o↵er the most promising experimental technique to reach the sensitivity required for the possible detection of a weak signal coming from the inter action of dark matter with the ordinary one. Liquid Argon (LAr), in par ticular, o↵ers an extraordinary and unique add-on key feature: an excellent discrimination power, to disentangle signal from the dominant electron-like background based on the scintillation pulse. The work presented here is performed in the framework of the DarkSide long term program, which aims at the WIMPs detection down to the neu trino floor, namely the irreducible background due to elastic scattering on nuclei by atmospheric neutrinos. DarkSide is a direct search dark matter program that developed the dark matter revelation technology by means of a Liquid Argon-based Time Projection Chamber (LAr TPC). DarkSide-50 (DS-50), in particular, is the currently running detector at INFN - Labora tori Nazionali del Gran Sasso (LNGS), Italy. It uses a dual phase LAr TPC filled with about 50 kg of liquid argon from underground sources. This is necessary because atmospheric argon contains a significant activity from the long-lived cosmogenic 39Ar. 39Ar is a beta-emitter which could contributes a significant amount of background and masks a possible dark matter signal in the detector. In the framework of DarkSide program, the R&D project ReD (Recoil Directionality) aims to investigate the realization of a liquid argon TPC to directly detect directionality signature, in the energy range of the expected WIMP-nucleus scattering recoils (up to 100 keV), by exploiting the recom bination e↵ect. Columnar recombination models suggest, in fact, that the magnitude of such a recombination e↵ect should vary with the angle between the electric field applied to the detector and the track direction. A di↵erence in the electron-ion recombination e↵ect is expected when the ionizing track is either parallel or perpendicular to the electric field. Recombination e↵ect was explored by the SCENE experiment, where a monoenergetic neutron beam was used to irradiate a small dual-phase LAr TPC with and without the application of an electric field. The collaboration achieved, for the first time, the comparison of the light and charge yield of recoils parallel and perpendicular with respect to the electric field (directional sensitivity), in case of a S1 signal due to scintillation. On the contrary, the same was not found for ionization signals. The ReD experiment was proposed with the aim to improve the SCENE measurements as part of the DarkSide program, so it may provide a fully scalable technology to bring the future dark matter experiments based on LAr to the multi-ton scale. The goal of the project is to irradiate a small LAr TPC with a neutron beam of known energy and direction by using the 15MV Tandem accelerator of the INFN - Laboratori Nazionali del Sud (LNS) in Catania, Italy. Neutrons are produced by means of the p(7Li,7Be)n two-body reaction in inverse kine matics, and then, thanks to the closed kinematics of the reaction, directed towards the TPC. Furthermore, most of the technological solutions adopted in ReD will be part of the future DarkSide-20k detector such as the cryogenic SiPMs that are, for the first time, tested in LAr with the ReD apparatus. In the following, the latest recents results on the characterization and the optimization of the ReD LAr TPC will be presented, together with a brief overview of the recent test beam performed in July 2019 at INFN - Labo ratori Nazionali del Sud in Catania. After a period of tests beams with the full ReD system during the 2018, a new physics run is scheduled early 2020