Characterization of argon recoils at the keV scale with ReD and ReD+

A key aspect in the direct search for light dark matter candidates with argon dual-phase time projection chambers (TPCs) is an accurate determination of the ionization yield (Qy) for low-energy nuclear recoils. While current direct measurements reach about 7 keV, extending the measurement to the sub-keV regime is crucial. The Recoil Directionality (ReD) experiment at the INFN Sezione di Catania, Italy, measured Qy for nuclear recoils down to 2 keV by irradiating a small dual-phase Ar TPC with neutrons from a $^{252}$Cf fission source. The ReD detector setup, data acquisition system, and analysis methodology are described, together with the results. The results are in agreement with existing data above 7 keV, while they indicate a higher Qy in the range 2-7 keV, which is probed for the first time using a direct and model-independent approach. The effort is being further extended by a new project, ReD+, at INFN Laboratori Nazionali del Sud. ReD+ is designed target nuclear recoils down to 0.2 keV, using improved components and a Deuterium-Deuterium generator.