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

Noemi Pino

Abstract

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.

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

Abstract

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.
Paper Structure (5 sections, 1 equation, 2 figures)

This paper contains 5 sections, 1 equation, 2 figures.

Table of Contents

  1. Introduction
  2. Conceptual design and experimental setup
  3. Data analysis and results
  4. The ReD+ phase
  5. Conclusions

Figures (2)

  • Figure 1: Sketch (not in scale) of the ReD experimental apparatus. From left to right, the setup includes the $^{252}$Cf neutron source surrounded by BaF$_2$ detectors inside the collimator shield, the dual-phase TPC, and the neutron spectrometer. The arrows indicate a neutron emitted from the source which undergoes (n,n') interaction with Ar in the TPC and is eventually scattered within the acceptance of the neutron spectrometer. See text and Ref. Agnes:2025rxi for more details.
  • Figure 2: Ionization yield measured in this work for nuclear recoils in the 2–10 keV energy range, including combined statistical and systematic uncertainties. For comparison, literature data up to 120 keV from Joshi et al. Joshi:2014fna, ARIS Agnes:2018mvl, and SCENE Cao:2015ks are also shown. The 6.7 keV data point from Joshi et al. has been rescaled following the procedure described in Ref. PhysRevD.104.082005. The energy region below 2 keV will be explored by the forthcoming ReD+ project (see Sect. \ref{['sec:redplus']}).