Heavy-element paleodetectors for Higgsino dark matter

Abstract: Paleodetectors have been proposed as a new approach to direct detection of weakly interacting massive particles (WIMPs), through the search for damage tracks in ancient minerals induced by WIMP-nucleon scattering. However, for inelastic dark matter such as the Higgsino, existing paleodetector targets lack sufficiently heavy nuclei to overcome the kinematic threshold for scattering. We propose heavy-element paleodetectors as a new probe of inelastic dark matter, using ancient, radiopure minerals containing heavy elements such as lead. We identify brine precipitates from deep geothermal aquifers as a possible geological source of such minerals. Additionally, paleodetectors are uniquely sensitive to the history of the dark matter high-velocity tail, including a possible fast population induced by the Large Magellanic Cloud's close approach 50 Myr ago. Such a scenario would favor younger minerals than usually assumed in the paleodetector literature. This method can probe Higgsino mass splittings up to $δ\simeq 920$ keV. Due to the large Higgsino-nucleon cross section, we find that even suboptimal mineral samples with ordinary radioactivity from depths of only 2 km can probe new parameter space, thus partially relaxing the stringent requirements on radiopurity and depth that constitute two significant challenges for the paleodetector program.

• The paper proposes a novel use of ancient Pb-rich minerals as passive nuclear track detectors to probe inelastic Higgsino dark matter, extending mass splitting sensitivity to ~920 keV.
• It employs detailed modeling of dark matter-nucleus interactions and high-velocity astrophysical distributions, notably from the LMC, to overcome kinematic limits of current xenon-based detectors.
• Advanced imaging techniques like SAXs and HIBM enable effective signal-background separation, reducing exposure needs while maintaining sensitivity in a low-background geological environment.

Heavy-Element Paleodetectors for Higgsino Dark Matter

Introduction and Motivation

The detection of dark matter (DM), particularly weakly interacting massive particles (WIMPs), remains a critical open problem at the interface of particle physics and cosmology. While direct detection experiments utilizing noble liquid targets (e.g., Xe, Ar) have set increasingly stringent limits on elastic WIMP-nucleon cross sections, large mass splitting scenarios—particularly those compatible with the supersymmetric Higgsino—are essentially unconstrained due to kinematic thresholds of available targets. The presented study proposes "heavy-element paleodetectors," specifically minerals with significant content of heavy nuclei like lead (Pb), as a new direction to address this gap. This approach targets inelastic DM, exploiting both the enhanced momentum reach of heavy targets and the unique long-term integration properties of paleodetectors.

Inelastic Dark Matter and Kinematic Access

The inelastic DM scenario postulates a splitting $\delta$ between two nearly degenerate mass states such that scattering through weak ($Z$-mediated) channels is off-diagonal and hence forbidden below a kinematic threshold set by $\delta$. The maximum accessible $\delta$ increases with both nucleus mass and incident DM velocity. For conventional xenon-based detectors, the target mass number limits sensitivity to $\delta \lesssim 350$ keV, while the heaviest existing paleodetector targets (iron) fare even worse. Figure 1 systematically maps the accessible recoil phase space for various mass splittings, highlighting the rapid kinematic closure as $\delta$ increases.

Figure 1: Contours of recoil energy $E_R$ for Pb nuclei as a function of DM velocity for various mass splittings $\delta$, with vertical dashed lines indicating maximal DM velocities for different astrophysical models.

To systematically push $\delta$ beyond this frontier, two avenues emerge: the use of heavier targets (Pb, Bi), and leveraging DM populations with higher velocities than the standard halo model (SHM) predicts.

Astrophysical Models and the Large Magellanic Cloud's Role

Direct detection projections are sensitive to the highest-velocity DM particles present locally. While most analyses assume the SHM (truncated Maxwell-Boltzmann), recent simulations [LMC] indicate the Milky Way's interaction with the Large Magellanic Cloud (LMC) $\sim50$ Myr ago could have populated the high-velocity tail with transient excesses, enhancing detector reach for large $Z$0. Figure 2 juxtaposes the halo integrals for both SHM and LMC-motivated distributions, illustrating the extended velocity support in the latter.

Figure 2: Halo integrals for SHM and LMC models at present and pericenter, showing enhanced high-velocity tail under LMC influence.

This historicity implies that maximizing detection probability for large $Z$1 does not generically prefer the oldest minerals: instead, exposures coinciding with recent high-velocity epochs are optimal.

Paleodetector Concept and Heavy-Element Target Materials

Paleodetectors are ancient minerals used as passive track detectors: over geological timescales, nuclear recoils from rare events (WIMPs, neutrinos) can create persistent track structures in the material lattice, detectable a posteriori with high-resolution imaging techniques (e.g., SAXs, HIBM). To enable sensitivity to inelastic DM at high $Z$2, the study introduces Pb-rich minerals, with a focus on Laurionite (PbClOH), as candidate hosts. Laurionite satisfies stringent electrical resistivity requirements for track retention and contains a high mass fraction of Pb ($Z$3). Geological sources such as brine precipitates in deep radiopure aquifers are identified, and uranium concentrations as low as $Z$4 g/g are documented, maintaining sufficiently low track backgrounds.

Signal and Background Modeling

The DM-induced recoils are calculated, accounting for the full nuclear form factor, stopping power, and track length mappings. For recoiling Pb nuclei in Laurionite, pronounced Helm form factor features are evident in both the recoil spectrum and the track-length space. These features are largely washed out after accounting for experimental resolution, especially under SAXs readout (15 nm), but preserve some discriminatory power with HIBM (1 nm).

Backgrounds are dominated by irreducible contributions from $Z$5U chain $Z$6-decay (single-step recoils), fast neutrons from both spontaneous fission and $Z$7 reactions, and solar/atmospheric neutrinos. The required radiopurity and mineral age considerations are carefully analyzed. Notably, for the largest cross section of $Z$8-mediated Higgsinos ($Z$9), the signal far exceeds those backgrounds, enabling the relaxation of sample purity and depth constraints over traditional elastically-targeted searches.

Projected Sensitivity and Numerical Results

The principal results assert that heavy-element paleodetectors using Pb-rich minerals with benchmark radiopurity and $\delta$0 exposure can probe Higgsino dark matter with mass splittings up to $\delta$1 keV (LMC-driven maximum), far surpassing the $\delta$2 keV limit from current xenon targets. Figure 3 details the projected exclusion limits as a function of $\delta$3 for different mineral ages and astrophysical scenarios.

Figure 3: Projected limit on DM-nucleon cross section as a function of mass splitting $\delta$4 for a $\delta$5 Laurionite sample, contrasting SHM and LMC scenarios and marking the Higgsino benchmark.

Key numerical findings include:

• Under the standard halo model with Gyr-old minerals, maximal accessible $\delta$6 is $\delta$7 keV.
• Incorporating the LMC high-velocity population for a 50 Myr-old sample extends reach to $\delta$8 keV.
• Even samples with uranium background typical of the Earth's upper crust or obtained from as shallow as $\delta$9 depth remain sensitive to previously unconstrained Higgsino parameter space, a regime unreachable by previous paleodetector proposals.
• Exposure and data storage demands for SAXs-based readout can be relaxed by factors up to $\delta$0 while still surpassing elastic search limits, further broadening practical applicability.

Implications and Future Directions

The study demonstrates that heavy-element paleodetectors offer an immediately actionable path to extend direct detection sensitivity to the last unconstrained regions of supersymmetric WIMP parameter space, specifically relevant for inelastic Higgsino DM. The method's unique sensitivity to the history of the local DM velocity distribution motivates further interdisciplinary engagement with galactic dynamics and merger simulations, to refine the optimal target mineral age and possible sensitivity to even earlier high-velocity events. The proposal also lays the groundwork for efficient searches utilizing accessible geological samples and reduced data handling, potentially democratizing exposure access beyond deep-mine infrastructure.

Theoretically, the approach can be translated to various inelastic DM scenarios beyond the minimal Higgsino, including other electroweak multiplets, dark photon-mediated models, and those with electromagnetic dipole transitions.

Conclusion

This work establishes heavy-element paleodetectors as a potent probe for inelastic dark matter, especially the supersymmetric Higgsino, enabling direct sensitivity to DM mass splittings up to $\delta$1 keV—well beyond the reach of current direct detection technologies. Leveraging the combined advances in materials geoscience, nuclear track detection, and galactic archaeology, this framework substantially expands the experimental reach for the remaining parameter space of classic WIMP models and motivates key astrophysical inquiries into the temporal evolution of DM velocity substructure.