Improved liquid argon ionization model and its impact on the DarkSide low-mass WIMP search programme

Abstract: DarkSide-50 achieved leading WIMP limits down to 1.2 GeV/c2 with an ionization-only analysis, despite its small active mass of 46 kg compared to multi-ton noble-liquid detectors. Accurate modelling of the nuclear-recoil ionization yield (Qy) is central to interpreting such searches. A new global fit combining nuclear-recoil response measurements from DarkSide-50, ARIS, SCENE, and the recent ReD experiment constrains Qy between 0.4 and 150 keV within the Thomas-Imel box framework. The dependence on screening potentials is addressed through a Bayesian model comparison, which rejects the ZBL and Molière functions and strongly favours the Lenz-Jensen one. The resulting model improves DarkSide-50 sensitivity below 3 GeV/c2 and refines the DarkSide-20k sensitivity accordingly.