DarkSide-50 has demonstrated the high potential of dual-phase liquid argon time projection chambers in exploring interactions of WIMPs in the $\mathrm{GeV}/{\mathrm{c}}^2$ mass range. The technique, based on the detection of the ionization signal amplified via electroluminescence in the gas phase, allows us to explore recoil energies down to the sub-keV range. We report here on the DarkSide-50 measurement of the ionization yield of electronic recoils down to $\sim 180{\mathrm{eV}}_{\mathrm{er}}$, exploiting ${}^{37}\mathrm{Ar}$ and ${}^{39}\mathrm{Ar}$ decays, and extrapolated to a few ionization electrons with the Thomas-Imel box model. Moreover, we present a model-dependent determination of the ionization response to nuclear recoils down to $\sim 500{\mathrm{eV}}_{\mathrm{nr}}$, the lowest ever achieved in liquid argon, using in situ neutron calibration sources and external datasets from neutron beam experiments.

• Received 20 July 2021
• Accepted 15 September 2021

DOI:https://doi.org/10.1103/PhysRevD.104.082005