New limits on spin-independent and spin-dependent couplings of low-mass WIMP dark matter with a germanium detector at a threshold of 220 eV

TL;DR

This work probes low-mass WIMPs using a four-channel ultra-low-energy germanium detector with a threshold of $220\pm10$ eV at KS, achieving $0.338$ kg·d exposure. By applying PSD, ACV/CRV vetoes, and an unbinned optimal-interval method, it sets new limits on both spin-independent WIMP-nucleon interactions and spin-dependent WIMP-neutron couplings in the sub-10 GeV mass range, notably improving constraints for $m_\chi$ between 3–6 GeV. It also provides a detailed treatment of trigger efficiency, quenching-factor modeling, and selection-cuts machinery, addressing criticisms and laying groundwork for scaling to kg-scale detectors. The results demonstrate the viability of sub-keV detectors to explore the low-mass WIMP parameter space and highlight the potential gains from larger, low-background, sub-keV experiments in the near term.

Abstract

An energy threshold of (220+-10) eV was achieved at an efficiency of 50% with a four-channel ultra-low-energy germanium detector each with an active mass of 5 g. This provides a unique probe to WIMP dark matter with mass below 10 GeV. With a data acquisition live time of 0.338 kg-day at the Kuo-Sheng Laboratory, constraints on WIMPs in the galactic halo were derived. The limits improve over previous results on both the spin-independent WIMP-nucleon and the spin-dependent WIMP-neutron cross-sections for WIMP mass between 3-6 GeV. Sensitivities for full-scale experiments are projected. This detector technique makes the unexplored sub-keV energy window accessible for new neutrino and dark matter experiments. Two appendices are included after Version-3 to address questions raised on trigger and selection efficiencies as well as other issues.

Figures (13)

• Figure 1: Schematic layout of the ULEGe with its anti-Compton detectors, inner shieldings and radon purge system. A 50-ton shielding structuretexonomagmom encloses the detectors.
• Figure 1: Typical $\rm{SA_{6}}$ pulses due to (a) Top: a random trigger event and (b) Bottom: a physics event at 139 eV in coincidence with anti-Compton signal. The pedestal mean and the discriminator threshold were denoted by dotted and dashed lines, respectively.
• Figure 1: Scattered plots of the $\rm{SA^{P}_{6}}$ versus $\rm{SA^T_{12}}$ signals, for both calibration and physics events before ACV and CRV cuts. The PSD selection is shown.
• Figure 2: (a) Top: Measured energy spectrum of the ULEGe with $^{55}$Fe source together with X-ray peaks from various materials. The black histogram represents events selected by PSD cuts. Deviations from the expected flat spectra contribute to PSD efficiencies. (b) Bottom: Scattered plots of the $\rm{SA^{P}_{6}}$ versus $\rm{SA^T_{12}}$ signals, for both calibration and physics events. The PSD selection is shown.
• Figure 2: The distributions of noise fluctuation of RT-events as well as of the maximum amplitudes of PHY-events in various energy bins. The discriminator threshold level is also shown.