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

TL;DR

This paper presents a new model for nuclear-recoil ionization yield in liquid argon, improving dark matter search sensitivity at low WIMP masses by integrating multiple experimental data and Bayesian model comparison.

Contribution

It introduces a global fit for ionization yield using diverse measurements and Bayesian analysis to select the best screening potential model, enhancing dark matter detection sensitivity.

Findings

Rejects ZBL and Molière screening functions based on Bayesian comparison.

Improves DarkSide-50 sensitivity below 3 GeV/c2.

Refines DarkSide-20k sensitivity estimates.

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 Moli\`ere 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.