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

Davide Franco

TL;DR

The paper addresses the challenge of interpreting low-mass WIMP searches in liquid-argon detectors by refining the nuclear-recoil ionization yield, $Q_y$. It performs a global fit to DarkSide-50, ARIS, SCENE, and ReD data within the Thomas–Imel box framework and tests competing nuclear screening functions, finding decisive support for the Lenz–Jensen model. The constrained $Q_y$ range of $0.4$–$150$ keV combined with Bayes-factor results, $ ext{log}_{10} ext{BF}=3.8$ against ZBL and $ ext{log}_{10} ext{BF}=7.2$ against Molière, establishes Lenz–Jensen as the preferred screening function. The updated ionization model enhances DarkSide-50 limits below $3$ GeV/$c^{2}$ and substantially boosts DarkSide-20k projections for sub-5 keV recoils, strengthening the program’s reach for low-mass WIMPs.

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.

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

TL;DR

The paper addresses the challenge of interpreting low-mass WIMP searches in liquid-argon detectors by refining the nuclear-recoil ionization yield, . It performs a global fit to DarkSide-50, ARIS, SCENE, and ReD data within the Thomas–Imel box framework and tests competing nuclear screening functions, finding decisive support for the Lenz–Jensen model. The constrained range of – keV combined with Bayes-factor results, against ZBL and against Molière, establishes Lenz–Jensen as the preferred screening function. The updated ionization model enhances DarkSide-50 limits below GeV/ and substantially boosts DarkSide-20k projections for sub-5 keV recoils, strengthening the program’s reach for low-mass WIMPs.

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.
Paper Structure (2 sections, 1 equation, 3 figures)

This paper contains 2 sections, 1 equation, 3 figures.

Table of Contents

  1. Ionization response of liquid argon to low-energy nuclear recoils
  2. Impact on DarkSide-50 limits and DarkSide-20k sensitivity

Figures (3)

  • Figure 1: Reduced nuclear stopping power as a function of reduced energy (bottom axis) and nuclear recoil energy in liquid argon (top axis), for the three screening potentials adopted in this work: ZBL, the Molière model implemented with the Wilson parametrization, and the Lenz–Jensen function.
  • Figure 2: Simultaneous fit to the ReD, ARIS, SCENE, and DarkSide-50 datasets assuming the Lenz–Jensen screening (green). ReD points are shown with prior (gray) and posterior (red) uncertainties. The previous ZBL-based model from ref. DarkSide:2021bnz is shown in gray, and global fits using ZBL (orange) and Molière (purple) screening functions are overlaid for comparison DarkSide-50:2025lns.
  • Figure 3: Updated 90% C.L. exclusion limits for DarkSide-50 (solid red) and projected sensitivities for DarkSide-20k (solid teal), obtained with the improved LAr ionization model based on the Lenz--Jensen screening function. Left: no-quenching-fluctuation (NQ) scenario. Right: quenching-fluctuation (QF) scenario. Previous ZBL-based limits and leading results from other experiments are shown for comparison DarkSide-50:2025lns.