Astronomical evidence of many types, including cluster and galaxy rotation curves, lensing studies and spectacular observations of galaxy cluster collisions, and cosmic microwave background (CMB) measurements, all point to the existence of cold dark matter (CDM) particles. Cosmological simulations based on the CDM model have been remarkably successful at predicting the actual structures we see in the Universe.
Alternative explanations involving modification of general relativity have not been able to explain this large body of evidence across all length scales.
Recent results from Gaia showing consistency with many previous experiments therefore much more securely pinned down than ever on: (a) DM dominates the mass of the Milky Way Galaxy, and (b) the local DM mass density in the Solar system is around 0.3 GeV/c2 · cm¯3.
Weakly Interactive Massive Particles (WIMPs) and Axions are among the two most prominent DM candidates. WIMPs are a hypothesized class of DM particles that would freeze out of thermal equilibrium in the early Universe with a relic density that matches observation. The so-called “WIMP miracle” is the coincident emergence of WIMPs with similar characteristics both from the solution of the gauge hierarchy problem and from the observed relic density of dark matter. Axions are motived by the strong Charge Parity (CP) problem, essentially to solve a fine-tuning problem of 1 part in ∼ 1010. Axions are much lighter and require different experimental techniques than WIMPs to detect them as in the Axion Dark Matter eXperiment (ADMX).
There are several viable strategies to detect DM. Indirect detection experiments aim to observe high-energy particles resulting from the self-annihilation of DM. Collider experiments look for the production of DM particles in high energy collisions. Direct detection experiments aim to observe the rare scatters of DM on the low threshold, very low background detectors operated in deep underground Laboratories. Table I categorizes currently active DM experiments.
The low-mass WIMPs region (100s MeV/c2 – 10 GeV/c2 ) has not been fully exploited comparing to high-mass WIMPs (10 GeV/c2 – 1 TeV/c2 ) 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 the recently published paper in Arxiv, Liao and colleagues have gone 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.
INTRODUCTION TO ALETHEIA
The ALETHEIA project is an instrumental background-free experiment with the ROI (Research Of Interest) of ∼ 100s MeV/c² – 10 GeV/c². With the S2 only analysis, the ALETHEIA project could be sensitive to 10s MeV/c² WIMPs.
Here, instrumental background-free means in the range of ROI, a small number of background events (for instance, < 0.1 events) are expected, such that zero background events are observed. The instrumental backgrounds include radioactive particles due to the materials in the detector system (including dust) and the particles generated by cosmological muons hitting the rocks near the detector or the detector itself. For low-mass WIMPs searches, there is another background which is registered by 8B solar neutrinos. The 8B events can’t be discriminated from low-mass WIMPs signals. Consequently, in a WIMPs detector that is free of instrumental background like the ALETHEIA, by subtracting (theoretically estimated) 8B events from the observed events, we can figure out how many WIMPs events were observed and this quantity’s uncertainty.
None of the leading high-mass DM experiments is sensitive to low-mass WIMPs due to their relative heavy target material, xenon, or argon. The ALETHEIA was inspired by both high-mass and low-mass WIMPs experiments in the community. Although there existed quite a few low-mass DM experiments, researchers believe the ALETHEIA is a competitive project in the race of hunting for low-mass DM thanks to the following advantages.
(a) Being able to discriminate Electron Recoil (ER) and Nuclear Recoil (NR) (ER/NR) events with ”S1/S2” analysis (where “S1” refers to prompt scintillation while “S2” is the electroluminescence light originated from an ionization by recoiled target nuclei) and possibly PSD ( Pulse Shape Discrimination ) technologies.
(b) LHe (liquid helium) could achieve extremely low or even zero intrinsic backgrounds. At 4 K temperature, 3He is the only solvable material in LHe while it is very rare in Nature; any other impurities would show in the solid-state. Impurities are supposed to be purified completely with getter and cold-trap technologies.
(c) 4He only has two nucleons and four electrons. Therefore the intrinsic ER background induced by (background) gammas and neutrinos would be significantly smaller than heavier noble elements like Ar and Xe.
(d) TPC is a technology demonstrated for LXe and LAr; it could possibly be employed on LHe. If an LHe TPC were made successfully, its fiducial volume is expected to achieve very few or even zero backgrounds by self-shielding and other data-analyzing technologies. and
(e), being capable of scaling up to a ton or multi-tons size to fully touch down the B-8 neutrino floor.
THE DESIGN OF THE ALETHEIA PROJECT
Unlike earlier concepts like HERON, the ALETHEIA project will use liquid helium at ∼ 4 K, which is above the Lambda curve, instead of superfluid helium. W. Guo and D. N. McKinsey also proposed and simulated a liquid helium TPC hunting for DM in 2013 with “S1/S2” analysis, where “S1” refers to prompt scintillation while “S2” is the electroluminescence light originated from an ionization by recoiled target nuclei. Besides the “S1/S2” analysis, the ALETHEIA will also implement the S2O analysis to reach the sensitive WIMPS mass all the way down to 10s MeV/c².
The design of the dual-phase ALETHEIA has been significantly inspired by other successful experiments like DarkSide-50, LUX/LZ, PandaX, XENON-100, and XENON-1T, and others. The photon sensors would be SiPMs to achieve the highest possible photon detection efficiency. Figure 1 shows the schematic drawing of such a detector.
As shown in Fig. 1, the core of the ALETHEIA experiment is a dual-phase liquid helium TPC (in cyan). The TPC center is in pink representing the fiducial volume where extremely low or zero background is expected. On the top and bottom of the TPC are SiPMs (purple). The TPC was surrounded by a Gd-doped scintillator detector, which acts as a veto. The outmost is a water tank with a diameter of a few meters to shield neutrons and gammas outside of the detector system.
For neutrons that come from outside of the water tank, a few meters of water should thick enough to thermalize them. The ∼ half-meter Gd-doped liquid scintillator would capture thermalized neutrons. For neutrons originated from the TPC inside, it could be identified by the feature of multiple hits in the TPC and (or) liquid scintillator. Typical WIMPs signal would only have one hit registered due to a much lower coupling constant between WIMPs and helium nuclei than the strong interaction between a neutron and helium nuclei. For γs from outside of the water, the water tank can block them from entering the central detector; for γs from inside of the TPC, “S1/S2”, or PSD (Pulse Shape Discrimination), or a hybrid analysis combined both could discriminate from nuclear recoils induced by a neutron or a WIMPs.
4He is suitable for low-mass WIMPs search thanks to several advantages:
(1) High recoil energy. The same kinetic energy of incident WIMPs would result in greater recoil energy than any other heavier elements. Hydrogen is even lighter, but the quenching factor of Hydrogen is more than one order smaller than Helium at the recoil energy of ∼1 keV_nr. ( ‘_’ is base)
(2) The Quenching Factor (QF) of LHe is quite high. For instance, for a 16 keV nuclear recoil, the measured QF of LHe is ∼ 65%; while LAr is ∼ 24%, which is a factor of 3 smaller. The measured Quenching Factor (QF) of Helium at 1.5 keV recoil energy is up to 22%. As a comparison, the measured QF of Hydrogen at 100 keV is only 2%, and the estimated QF at 1.5 keV nuclear recoils would be much lower. As a result, the QF of Hydrogen is guaranteed to be at least one order smaller than 4He. Thus, Hydrogen is not an appropriate material for low-mass WIMPs search with the method of ionization, while Helium is.
(3) 4He only has 4 electrons. Therefore the intrinsic ER background induced by (background) gammas would be significantly smaller than other widely used heavier noble elements like Argon and Xenon.
(4) At 4 K, only 3He is solvable in LHe, and 3He is very few in nature; other elements become a solid state at this low temperature. So, it would be easy to purify LHe with getter and cold trap technologies to achieve very high level purification.
(5) LHe is relatively cheaper. The price of LHe is ∼ 1/7 of LXe. (3He is very expensive, which is partially the reason why it has been ruled out for consideration as a material for low-mass WIMPs search.).
First 30 g LHe detector
Although lots of theoretical and experimental research have been launched to understand the particle characters of LHe, most of those experimental results focused on 100s keV_nr energy or higher, which are out of the ROI of the proposed LHe TPC: ∼ 0.5 –10 keV_nr. Therefore, researchers have to launch a complete set of Experimental tests to verify that an LHe TPC is suitable for low-mass DM hunting. The first 30 g LHe detector is assembled at CIAE in Beijing, China, and they are ready to launch tests soon.
Featured image: The mechanical drawing of their first 30 g LHe cell. The left plot shows the test bench. The right one is the internal structure of the whole 30 g LHe cell. The unit for “D:130” and “H:42” on the right plot is mm. © Liao et al.
Reference: Junhui Liao, Yuanning Gao, Zhuo Liang, Zhaohua Peng, Lifeng Zhang, Lei Zhang, “A low-mass dark matter project, ALETHEIA: A Liquid hElium Time projection cHambEr In dArk matter”, ArXiv, pp. 1-32, 2021. https://arxiv.org/abs/2103.02161
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