Probing Cosmic-Ray-Boosted and Supernova-Sourced Sub-GeV Dark Matter with Paleo-Detectors

This paper demonstrates that paleo-detectors using olivine targets can achieve unprecedented sensitivity to sub-GeV dark matter accelerated by cosmic rays and supernovae, enabling the detection of nuclear recoil tracks over geological timescales and probing parameter regions inaccessible to current and near-future experiments.

The Gist

The Big Picture: Hunting the "Ghost" that Moves Too Slowly

Imagine you are trying to catch a ghost. But this isn't a spooky ghost; it's Dark Matter, the invisible stuff that makes up most of the universe.

For decades, scientists have been trying to catch these ghosts using giant tanks of liquid xenon buried deep underground. They wait for a ghost to bump into an atom in the tank, hoping to see a tiny flash of light. This works great for "heavy" ghosts (like WIMPs).

But what if the ghosts are actually tiny and light (sub-GeV)?

• The Problem: In our galaxy, these light ghosts move very slowly. When they hit a heavy atom, they bounce off like a ping-pong ball hitting a bowling ball. The bowling ball doesn't even notice. The energy transfer is so small that our current detectors can't see it. It's like trying to hear a whisper in a hurricane.

The Solution: Giving the Ghosts a "Speed Boost"

The authors of this paper propose a clever workaround. They suggest that some of these light ghosts might get a speed boost from two cosmic sources:

1. Cosmic Rays: High-energy particles from space that smash into dark matter, kicking them like a soccer ball.
2. Supernovas: Exploding stars that act like giant particle accelerators, shooting dark matter out at near-light speeds.

Now, instead of a slow-moving ping-pong ball, we have a bullet. When this "boosted" dark matter hits an atom, it creates a big enough crash to be detected.

The Detector: The "Geological Hard Drive"

Here is where the paper gets really creative. Instead of building a bigger, more expensive machine, the authors suggest using ancient rocks.

• The Analogy: Imagine a pristine, white sheet of paper. If you run a tiny, sharp needle across it, it leaves a microscopic scratch.
• The Rock: They propose using Olivine, a green mineral found in Earth's mantle (deep underground).
• The Process: Over millions of years, if a "boosted" dark matter bullet hits an atom inside the olivine crystal, it knocks the atom loose. That atom then plows through the crystal lattice, leaving a tiny, permanent scratch (or track).
• The Superpower: A normal detector runs for a few years. An ancient rock has been running for billions of years.
  • If you have 100 grams of rock that has been sitting there for 1 billion years, it has the same "listening time" as a 100,000-ton detector running for a year.
  • It's like listening to a radio station for a billion years to catch one rare song, rather than listening for one hour.

The "Noise" Problem

Of course, rocks aren't perfect. They have "noise" (background interference):

• Radioactivity: Uranium in the rock decays and creates scratches.
• Neutrinos: Ghostly particles from the Sun and the atmosphere that also leave scratches.
• Defects: Natural cracks in the rock.

The authors did the math to figure out how to tell the difference between a "Dark Matter scratch" and a "Radioactive scratch." They found that because the boosted dark matter moves so fast, it leaves scratches of a specific length and shape that are different from the background noise.

The Results: A New Superpower

The paper calculates that if we take a 100-gram sample of this ancient olivine and look at it under a powerful microscope (using X-rays or electron beams) to count the scratches, we could:

1. See the Invisible: Detect dark matter that is too light and too fast for current experiments like XENON or LUX.
2. Supernova Time Capsule: This is the coolest part. Because the rock has been recording for billions of years, it might contain a "fossil record" of dark matter that was shot out by supernovas that exploded millions of years ago. We could essentially see the "echo" of ancient stellar explosions that no human has ever witnessed.

The Bottom Line

This paper suggests that we don't need to build a bigger machine to find light dark matter. We just need to find a really old rock, dig it out from deep underground, and look at it under a microscope.

It turns the Earth itself into a giant, billion-year-old camera that has been taking pictures of the universe's most elusive particles, waiting for us to develop the technology to look at the photos.

In short: We are using the Earth's history to catch the ghosts that our modern machines are too small to see.

Technical Summary

1. Problem Statement

Direct detection of sub-GeV Dark Matter (DM) is challenging for conventional experiments (e.g., XENON, PandaX) because the typical Galactic DM velocity ($v \sim 10^{-3}c$) results in nuclear recoil energies below the keV detection threshold when scattering off heavy nuclei. While astrophysical acceleration mechanisms can boost DM to semi-relativistic speeds, the resulting flux is significantly lower than the standard virialized DM halo, requiring massive exposure to detect.
The Core Challenge: How to achieve sufficient sensitivity to detect the rare, boosted sub-GeV DM flux (from Cosmic Rays or Supernovae) without relying on the massive target masses of current experiments, which are limited by cost and background noise.

2. Methodology

The authors propose using Paleo-Detectors—ancient minerals (specifically olivine, $(Fe, Mg)_2SiO_4$) that record nuclear recoil damage tracks over geological timescales (Gigayears).

• Target Material & Exposure: The study assumes 100 grams of olivine exposed for 1 Gyr, yielding an effective exposure of $\sim 10^5$ ton$\cdot$yr, far exceeding current direct detection experiments.
• Acceleration Mechanisms:
  1. Cosmic-Ray-Boosted DM (CRDM): DM particles gain energy via elastic scattering with high-energy cosmic rays (CRs). The authors calculate the flux using a constant spin-independent cross-section ($\sigma_{\chi p}^{SI}$) and account for Earth's attenuation (scattering through 5 km of crust).
  2. Supernova-Sourced DM (SNDM): DM is thermally produced in the hot cores of core-collapse supernovae (SN) and escapes. This is modeled using a Dark Photon mediator framework. The flux follows a Fermi-Dirac distribution peaking around the SN temperature ($T \sim 30$ MeV). The authors consider both "free-streaming" and "trapping" regimes based on the coupling strength.
• Signal Simulation:
  • Calculated differential nuclear recoil rates.
  • Mapped recoil energy ($E_R$) to track length ($x$) in olivine using SRIM software, accounting for nuclear and electronic stopping powers.
  • Generated track length distributions for various DM masses (MeV to hundreds of MeV).
• Background Modeling: The analysis incorporates major backgrounds with Gaussian uncertainties:
  • Solar, Diffuse Supernova (DSNB), and Galactic Supernova (GSNB) neutrinos.
  • Atmospheric neutrinos.
  • Neutrons from $^{238}$U decay chains.
  • $^{234}$Th recoils.
  • Note: The analysis is restricted to track lengths $x < 10^3$ nm to avoid intrinsic crystal defects.
• Statistical Analysis: A binned likelihood analysis using an Asimov dataset (background-only hypothesis) was performed to derive 95% Confidence Level (C.L.) exclusion limits on the DM-nucleon cross-section.

3. Key Contributions

• First Application to Sub-GeV Boosted DM: While paleo-detectors have been studied for GeV-scale WIMPs, this is the first comprehensive study applying them to accelerated sub-GeV DM via CRDM and SNDM mechanisms.
• Conservative Background Treatment: Unlike previous studies that might include long tracks, this work restricts analysis to short tracks ($< 1000$ nm) and treats all background uncertainties conservatively, ensuring robust sensitivity projections.
• SNDM vs. CRDM Comparison: The paper provides a rigorous comparison between the two acceleration mechanisms, revealing that the SNDM mechanism offers significantly stronger constraints due to the high flux of DM produced in supernovae.
• Flux Attenuation Modeling: Detailed calculation of how Earth's crust attenuates the CR-boosted DM flux, showing that for cross-sections $\sigma \gtrsim 10^{-29}$ cm$^2$, the signal is significantly suppressed before reaching the detector.

4. Key Results

• Sensitivity Projections:
  • CRDM: Paleo-detectors with 100 g$\cdot$Gyr exposure can probe spin-independent cross-sections down to $\sim 10^{-32}$ cm$^2$ for DM masses around 10–100 MeV. This surpasses current and near-future experiments (XENON, PandaX, Super-K) in the sub-GeV mass range.
  • SNDM: The sensitivity is dramatically higher. Paleo-detectors can probe cross-sections ($\sigma_{\chi p}$) as low as $\sim 10^{-47}$ cm$^2$.
• Magnitude of Improvement:
  • For the SNDM scenario, the sensitivity is 5 to 10 orders of magnitude stronger than that derived from CR-boosted DM signals.
  • In the mass range of $O(1)$ to $O(100)$ MeV, paleo-detectors provide the strongest constraints on DM-nucleon interactions compared to all other proposed experiments (including Hyper-K, DUNE, DARWIN, and LZ).
• Track Length Distributions:
  • CRDM signals show a dependence on DM mass where heavier DM produces longer tracks due to higher energy transfer efficiency.
  • SNDM signals exhibit a more pronounced long-track tail because the SN-sourced flux peaks at high kinetic energies ($\sim 100$ MeV) regardless of the DM mass (as long as $m_\chi \lesssim T_{SN}$).

5. Significance

• Unique Discovery Potential: Paleo-detectors offer a unique capability to integrate the DM flux from past Galactic supernova events over geological timescales. This cumulative exposure allows for the detection of rare signals that would be invisible to short-duration, real-time experiments.
• Complementarity: This approach complements traditional direct detection by targeting a specific mass range (sub-GeV) and interaction type that is currently unconstrained or poorly constrained by other methods.
• Feasibility: The results suggest that if sub-GeV DM exists and is accelerated by astrophysical sources, ancient minerals could already contain the evidence. With improvements in track readout resolution (X-ray, ion-beam, or electron microscopy) and background reduction, paleo-detectors represent a viable, high-sensitivity tool for the next generation of dark matter searches.

In conclusion, the paper demonstrates that paleo-detectors are not only theoretically capable but potentially superior to current technology for probing the parameter space of sub-GeV dark matter, particularly when sourced from supernovae.