Extending direct measurements of argon nuclear recoils into the sub-keV regime with ReD and ReD+

Direct searches for dark matter in the form of WIMPs with argon-based detectors require precise measurements of the ionization yield \Qy\ for nuclear recoils at low energies. Prior to this work, direct experimental data were available only above 6.7 keV, leaving a critical gap in the energy region most relevant for low-mass WIMP searches. The Recoil Directionality (ReD) experiment addressed this limitation by measuring the argon \Qy\ for nuclear recoils between 2 and 10 keV using a dual-phase TPC irradiated with neutrons from a \Cf\ fission source. The results extend existing direct measurements to lower energies, show consistency with previous data above 7 keV, and indicate an enhanced ionization yield at low recoil energies. These measurements provide essential input for next-generation argon-based dark matter searches and directly motivate the upgraded ReD+ phase, designed to further extend sensitivity into the sub-keV recoil-energy regime.