SENSEI: Direct-Detection Constraints on Sub-GeV Dark Matter from a Shallow Underground Run Using a Prototype Skipper-CCD

Abstract: We present new direct-detection constraints on eV-to-GeV dark matter interacting with electrons using a prototype detector of the Sub-Electron-Noise Skipper-CCD Experimental Instrument. The results are based on data taken in the MINOS cavern at the Fermi National Accelerator Laboratory. We focus on data obtained with two distinct readout strategies. For the first strategy, we read out the Skipper-CCD continuously, accumulating an exposure of 0.177 gram-days. While we observe no events containing three or more electrons, we find a large one- and two-electron background event rate, which we attribute to spurious events induced by the amplifier in the Skipper-CCD readout stage. For the second strategy, we take five sets of data in which we switch off all amplifiers while exposing the Skipper-CCD for 120k seconds, and then read out the data through the best prototype amplifier. We find a one-electron event rate of (3.51 +- 0.10) x 10^-3 events/pixel/day, which is almost two orders of magnitude lower than the one-electron event rate observed in the continuous-readout data, and a two-electron event rate of (3.18 +0.86 -0.55) x 10^-5 events/pixel/day. We again observe no events containing three or more electrons, for an exposure of 0.069 gram-days. We use these data to derive world-leading constraints on dark matter-electron scattering for masses between 500 keV to 5 MeV, and on dark-photon dark matter being absorbed by electrons for a range of masses below 12.4 eV.

• The paper introduces a novel Skipper-CCD method that detects single-electron events from sub-GeV dark matter with unprecedented low-noise precision.
• It employs continuous and periodic readout strategies to measure dark matter-electron scattering, setting constraints for masses between 500 keV and 5 MeV.
• The results pave the way for improved dark matter experiments through advancements in CCD technology and deeper underground deployments.

SENSEI: Direct-Detection Constraints on sub-GeV Dark Matter from a Prototype Skipper-CCD

The SENSEI collaboration presents significant advancements in the direct detection of sub-GeV dark matter (DM), utilizing a prototype Skipper-Charge-Coupled Device (Skipper-CCD) placed in the MINOS cavern at Fermi National Accelerator Laboratory. This paper explores detection capabilities for dark matter-electron interactions and introduces direct-detection constraints for dark matter masses between eV and GeV scales. The utilization of Skipper-CCD technology in the setup enhances sensitivity to single electron events, making it particularly advantageous for probing low-mass dark matter candidates.

Methodology

The SENSEI apparatus comprises a silicon Skipper-CCD with around one million pixels, optimized for detecting electron excitations due to interactions with hypothetical dark matter particles. This study focused on eV-to-GeV mass-scale dark matter candidates—an area previously less accessible to direct-detection strategies primarily targeting heavier Weakly Interacting Massive Particles (WIMPs). Skipper-CCD technology allows for substantially lower noise levels, enabling the counting of electrons on a per-pixel basis with unprecedented precision.

The research involved two distinct readout strategies: continuous and periodic. The continuous-readout method, though indicating high background rates of single and two-electron events, was crucial in analyzing more significant events (three or more electrons). The periodic-readout, with amplifiers shut off during exposure and activated post-exposure, yielded a drastically reduced background, allowing for sensitive detection limits of one- and two-electron events.

Results

The paper reports an effective constraint on dark matter-electron scattering for dark matter masses ranging from 500 keV to 5 MeV. For instance, the one-electron event rate was deduced to be $(3.51 \pm 0.10) \times 10^{-3}$ events/pixel/day—nearly two orders of magnitude lower than in the continuous-readout, thereby reducing the noise impact considerably. Furthermore, no significant electron discharge was observed for three or more electrons in 0.246 gram-days of combined exposure, strengthening the derived constraints.

Implications and Future Work

These findings represent a substantial step toward the direct detection of lower-mass dark matter, offering world-leading constraints and enhancing the physics community's understanding of sub-GeV dark matter-electron interactions. The techniques outlined in this research could improve the modeling of dark matter, contributing to theoretical frameworks necessary for understanding dark-sector physics.

The study suggests several future paths, including improvements in device design and electronic noise mitigation strategies. The collaboration aims to scale up the experiment, with enhanced Skipper-CCDs planned for deployment at deep-underground laboratories like SNOLAB. Characterization of the dark matter background environment and refining CCD technologies will be crucial for forthcoming experiments, potentially leading to groundbreaking discoveries regarding the fundamental nature of dark matter.

Conclusion

The SENSEI collaboration has set a precedence in the low-mass dark matter detection domain, tapping into new observational thresholds with enhanced technology and methodologies. The depth and robustness of the results provide a template for future direct-detection dark matter experiments, auguring well for forthcoming empirical validations in the ongoing search for dark matter.