A low-mass dark matter project, ALETHEIA: A Liquid hElium Time projection cHambEr In dArk matter

Abstract: Dark Matter (DM) is one of the most critical questions to be understood and answered in fundamental physics today. Observations with varied astronomical and cosmological technologies already pinned down that DM exists in the Universe, the Milky Way, and the Solar System. However, the understanding of DM under the language of elementary physics is still in progress. DM direct detection aims to test the interactive cross-section between galactic DM particles and an underground detector's nucleons. Although Weakly Interactive Massive Particles (WIMPs) is the most discussed DM candidate, the null-WIMPs conclusion has been consistently addressed by most convincing experiments in the field. The low-mass WIMPs region (100s MeV/c$^2$ - 10 GeV/c$^2$) has not been fully exploited comparing to high-mass WIMPs (10 GeV/c$^2$ - 1 TeV/c$^2$) experiments which implement liquid xenon or argon TPCs (Time Projection Chambers). The ALETHEIA experiment aims to hunt for low-mass WIMPs with liquid helium-filled TPCs. In this paper, we go through the physics motivation of low-mass DM, the ALETHEIA detector's design, a series of R&D programs that should be launched to address a liquid helium TPC's functionality, and possible analysis channels available for DM searches. We designed and homemade a 30 g liquid helium prototype detector at CIAE. In Summer 2021, the detector was cooled down to 4.5 K successfully. The detector's dark current is less than ten pA for an external voltage up to 17 kV/cm when filled with vacuum, 1 atm nitrogen gas, and liquid nitrogen.