Direct Search for Low Mass Dark Matter Particles with CCDs

TL;DR

This work demonstrates a direct dark matter search targeting low-mass candidates by leveraging thick, fully-depleted high-resistivity CCDs with an ultra-low energy threshold of $40~\mathrm{eV_{ee}}$. The approach combines meticulous CCD calibration and diffusion-based event discrimination to isolate nuclear-recoil-like signals, followed by an underground engineering test that sets competitive limits for $m_\chi \lesssim 4$ GeV. The study identifies neutron-dominated backgrounds in shallow sites as a major hurdle and outlines a clear path to significant sensitivity improvements via deeper underground deployment, neutron shielding, and future skipper-CCD upgrades that could dramatically reduce readout noise and energy thresholds. Overall, the results establish CCD-based direct detection as a viable technology for probing the low-mass dark matter parameter space and chart practical steps toward deeper, lower-background experiments (e.g., at SNOLAB).

Abstract

A direct dark matter search is performed using fully-depleted high-resistivity CCD detectors . Due to their low electronic readout noise (RMS ~ 7 eV) these devices operate with a very low detection threshold of 40 eV, making the search for dark matter particles with low masses (~ 5 GeV) possible. The results of an engineering run performed in a shallow underground site are presented, demonstrating the potential of this technology in the low mass region.

Figures (13)

• Figure 1: Four muons tracks are clearly reconstructed on this image. There is a significant width different between the two ends of the track. The thicker end corresponds to the region of the track in the back of the CCD, with maximum diffusion. The thinner end corresponds to the front of the CCD. The smaller circular hits are diffusion-limited hits as expected for nuclear recoils. The line is 100 pixels long and was added to give a sense of scale.
• Figure 2: Spectrum obtained for the reconstructed X-ray hits in an $^{55}$Fe exposure of a DECam CCD. The arrows mark the direct X-rays from the source: K$\alpha$=5.9 keV and K$\beta$=6.5 keV, their escape lines at 4.2 and 4.8 keV janesick, and the Si X-ray at 1.7 keV. The factor 3.64 eV/e is used to convert from charge to ionization energy. The feature at 7.6 keV is consistent with pixels that hit by a 5.9 keV and a 1.7 keV X-ray on the same exposure. consistent w
• Figure 3: Existing quenching factor measurements in Si compared with the Linhard theory (solid line). No previous data exist for recoil energies below 4 keVLindhardChagani.
• Figure 4: Image resulting from an exposure of a DECam CCD to a $^{252}$Cf neutron source. The total width of the image corresponds to 1000 pixels. The smaller dots represent the diffusion-limited hits, the trails correspond to scattered electrons and there is one bigger circular cluster of charge that corresponds to an alpha particle.
• Figure 5: Reconstructed electron equivalent energy spectrum for $^{252}$Cf exposures. The data is consistent with expectations from Lindhard theory (red). The expectations for an energy independent quenching factor are also shown for comparison (green).