Sensitivity to low-mass WIMPs with an improved liquid argon ionization response model within the DarkSide programme

Abstract

Dark matter detection experiments using liquid argon rely on a precise characterization of the ionization response to nuclear recoils, especially in the keV energy range relevant for light dark matter interactions. In this work, we present a comprehensive analysis that combines new measurements from the ReD setup, part of the DarkSide experimental program, with calibration data from DarkSide-50, as well as results from the ARIS and SCENE experiments. These combined datasets enable improved constraints on atomic screening effects in the modeling of the ionization response of liquid argon to nuclear recoils. By including the updated ionization model into the DarkSide-50 analysis framework, we obtain stronger exclusion limits on low-mass WIMP interactions, setting new world-leading constraints in the 1-3 GeV/c$^2$ WIMP mass range. Finally, we recast the sensitivity projections for the upcoming DarkSide-20k detector, demonstrating a significantly enhanced discovery potential for low-mass dark matter candidates.

Figures (5)

• Figure 1: Simultaneous fit to the ReD, ARIS, SCENE, and DarkSide-50 datasets assuming the Lenz-Jensen screening function (green solid line). ReD data points are shown with both prior (gray) and posterior (red) uncertainties. The gray line and its corresponding uncertainty band represent the previous ionization model, based on the ZBL screening function and fitted without ReD data DarkSide:2021bnz. For comparison, the global fit including the ReD dataset was also performed using the screening functions of ZBL (orange dashed line) and Molière (purple dashed line).
• Figure 2: Probability density functions of the expected WIMP-induced ionization spectra in DarkSide-50 for WIMP masses of 1.2, 3.5, and 7.0 GeV/c$^2$, shown for three different SF models. The $f_q(E_{nr})$ response based on the ZBL SF corresponds to the previous fit without ReD data DarkSide:2021bnz, while the Molière SF-based $f_q(E_{nr})$ curve is from this work. In the top panel, no fluctuations are assumed in the nuclear recoil quenching process (NQ), whereas in the bottom panel, quenching fluctuations are modeled with a binomial distribution (QF).
• Figure 3: DarkSide-50 (red) exclusion limits with 4 $e^-$ analysis threshold and DarkSide-20k (teal) expected sensitivity assuming binomial quenching fluctuation model (QF), 10 years exposure, and 2 $e^-$ threshold. The updated limits are derived using the Lenz-Jensen screening function in the LAr ionization response model and are compared to previous results obtained with the ZBL screening function. Most recent limits from XENONTnT XENON:2024hup and PandaX-4T PandaX:2022aacPandaX:2022xqx are also presented. The neutrino fog in LAr with index n=2 OHare:2021utq is also shown.
• Figure 4: DarkSide-50 exclusion limits (red) and DarkSide-20k projected sensitivity (teal) under the NQ assumption. For color scheme and references, see the caption of Fig. \ref{['fig:limits_nq']}.
• Figure 5: Global fit under the ZBL screening model. The $1\sigma$ (solid line), $2\sigma$ (dashed), and $3\sigma$ (dotted) confidence contours are shown for each dataset, along with the global fit result (cyan).