Heavy Paleodetectors for Hunting the Higgsino: How Ancient Minerals Reveal the Secrets of Dark Matter ✦
Heavy-element paleo-detectors are proposed for the direct detection of inelastic dark matter, using the higgsino as a benchmark. Existing paleo-detector methods lacked sufficiently heavy nuclei to overcome the kinematic scattering threshold. Suitable minerals may be sourced from ancient low-background sediments (e.g., lead-rich) from deep geothermal brines. The method shows enhanced sensitivity to the high-velocity tail of the dark matter distribution, particularly to a possible fast-particle stream triggered by a close passage of the Large Magellanic Cloud ~50 Myr ago, making younger samples preferable. Accessible higgsino mass splittings range up to δ≈920 keV. Due to the large higgsino-nucleon scattering cross-section, even samples with typical background levels from depths of only 2 km can probe new parameter space, partially relaxing the strict requirements on radio-purity and depth that are critical for the paleo-detector program.
Context
Almost a century ago, astronomers Fritz Zwicky and Vera Rubin discovered that the motion of galaxies cannot be explained without additional invisible mass — dark matter. Today we know it makes up a quarter of the universe’s total energy and played a key role in structure formation after the Big Bang, as imprinted in the cosmic microwave background. However, its particles likely interact extremely weakly with ordinary matter. If dark matter consists of WIMPs, the hunt becomes a quest at the intersection of particle physics and cosmology. Of particular interest is the higgsino — the superpartner of the Higgs boson — which can scatter off nuclei with a large but inelastic cross-section, making it nearly invisible to conventional detectors. Like in a detective story, we turn to “stones that remember the past” — ancient minerals.
Methods
The key idea of paleodetectors is to use minerals that served as targets for dark matter particles for millions or billions of years. Each recoiling nucleus collision leaves a track in the crystal lattice, fractions of a micron long. To overcome the kinematic threshold of inelastic scattering, heavier nuclei are needed: for example, lead-207 in the mineral laurionite (PbClOH). This mineral forms in brines of deep geothermal aquifers, where hot water contains salts and metals. Thanks to reducing conditions created by the decomposition of organic matter rich in carbon, thorium and uranium remain virtually insoluble, making the mineral surprisingly pure — with uranium concentration as low as 3×10^{-12} g/g. Moreover, laurionite contains hydrogen and oxygen, drastically reducing the background from fast neutrons. After extraction, the mineral is “read” using either small-angle X-ray scattering (SAXS) or helium-ion microscopy (HIBM), where a beam of helium ions scans the sample. With SAXS, volumes up to 60 cm³ can be processed at 15 nm resolution; with HIBM, only 6 mm³, but with nanometer precision. Additionally, mass spectrometry helps accurately measure impurity levels.
Results
Calculations show that a laurionite-based paleodetector can detect higgsino scattering with a cross-section of about 10^{-39} cm² at a mass splitting of up to δ ≈ 920 keV in a scenario with a boosted dark matter stream due to the approach of the Large Magellanic Cloud (LMC) 50 million years ago. Without this effect, within the Standard Halo Model, δ ~ 560 keV is achievable. In an optimistic case with record-low uranium content (~10^{-13} g/g), the limit approaches the theoretical maximum for lead — 960 keV. Even under significantly worse conditions — 2 km depth and uranium contamination at the parts-per-million level — the method probes new parameter space. Younger minerals, 50 million years old, proved preferable to ancient ones because they accumulated signal only during the period of the fast dark matter component, minimizing background noise. A comparison of readout methods showed that HIBM becomes advantageous in high-background scenarios, as its nanometer resolution can distinguish narrow signal peaks.
Implications
This approach for the first time enables direct detection of inelastic dark matter with large cross-sections, orders of magnitude beyond the sensitivity of conventional experiments. It brings into reach one of the last “classic” WIMP candidates — the higgsino — whose scattering was kinematically forbidden for xenon detectors. Moreover, heavy-element paleodetectors may be the only way to test scalar electroweak WIMPs. The method also motivates studying the history of the dark matter speed tail in the Galaxy, linking astrophysical simulations with laboratory physics.
Future development
In the future, modeling collisions of the Milky Way with other dwarf galaxies could identify epochs with even faster dark matter particles, extending the sensitivity range. The search for suitable minerals — for example, lead and bismuth halides — may uncover natural samples with ultra-low radioactive impurity levels. Advances in three-dimensional track reconstruction with nanometer resolution will allow scanning larger volumes and searching for rare events.
Impact
The technology will impact particle physics (especially supersymmetric extensions of the Standard Model), dark matter astrophysics (dynamics of galactic encounters), and geology (study of deep-seated brines).
Next steps
Experimentalists are urged to promptly examine deep borehole cores (e.g., in the Gulf of Mexico region) for laurionite or similar minerals. Detailed simulations of dark matter velocity distributions in the past are also needed to better select the optimal sample age.
Key open problems
The work is directly connected to the unsolved problem of identifying the dark matter particle. It offers a workaround for detecting inelastic interactions that elude detectors based on light nuclei. It also addresses the influence of recent galactic mergers on the local dark matter population — an aspect of dark matter physics that remains poorly studied.
🎯 Interestingly, the best limits on uranium content in brines were obtained back in the 1980s during geothermal energy exploration. It turns out that last century’s technologies unexpectedly help in the search for dark matter!
🎬 A similar plot appears in Arthur C. Clarke’s novel “Rendezvous with Rama,” where characters study artifacts of ancient civilizations to understand cosmic laws. Here, we study nature’s “artifacts” — minerals that have been recording cosmic history for millions of years.
Key numbers
- max mass splitting: ~920 keV
- higgsino scattering cross-section: ~10^{-39} cm²
- optimal mineral age: 50 million years
- uranium concentration in pure sample: 3×10^{-12} g/g
- depth for ultra-clean samples: 5 km