Cosmic ray electron boosted light dark matter: Implications of LZ 2025 data

This paper demonstrates that using the latest LZ 2025 data to detect cosmic-ray-boosted light dark matter significantly improves constraints on sub-MeV electrophilic dark matter, surpassing previous XENONnT limits and probing unexplored mediator parameter spaces even against neutrino detector benchmarks.

The Gist

The Big Mystery: The Invisible Ghost

Imagine the universe is a giant, dark room filled with invisible "ghosts" called Dark Matter. We know they are there because they have gravity (they pull on stars and galaxies), but we can't see them, and they don't seem to bump into normal stuff like air or water.

For decades, scientists have built massive, ultra-sensitive detectors deep underground (like the LZ detector in the US) to catch these ghosts. They wait for a ghost to bump into an atom in the detector, creating a tiny flash of light.

The Problem:
Most of these ghosts are very heavy and slow-moving. But there's a theory that some might be very light (like a feather compared to a bowling ball). If a feather floats by a bowling ball, the bowling ball doesn't even notice. Similarly, these light ghosts move so slowly that when they hit the detector, they don't make enough energy to trigger the alarm. The detectors are too "heavy" to feel the "feather."

The Solution: The Cosmic "Slingshot"

This paper proposes a clever workaround. It suggests that while most dark matter is slow, a tiny fraction of it gets a boost from something else: Cosmic Rays.

Think of Cosmic Rays (specifically high-speed electrons) as a swarm of super-fast bullets flying through space.

1. Imagine a slow-moving dark matter ghost (the feather) floating in space.
2. A cosmic ray bullet (the electron) smashes into it.
3. This collision acts like a slingshot, flinging the dark matter ghost forward at incredible speeds.

Now, this "boosted" ghost is moving so fast that when it finally hits the detector, it creates a big enough splash to be seen.

What the Authors Did

The authors, Sk Jeesun and Anirban Majumdar, took the latest data from the LZ experiment (specifically data from a run called "WS2024") and asked: "If these boosted ghosts exist, could we have seen them already?"

They didn't just look for a simple bump; they looked at two different ways the "slingshot" might work, based on the physics of the invisible force carrying the energy (called a mediator):

1. The Heavy Mediator: Like a thick, solid bat. The collision is strong and direct.
2. The Light Mediator: Like a thin, stretchy rubber band. The force changes depending on how close the particles get.

The Results: A New Record

Here is what they found, using simple comparisons:

• Beating the Old Record: Previous experiments (like XENONnT) had set limits on how light these ghosts could be. The authors found that with the new LZ data, they can rule out a much wider range of possibilities. They improved the constraints by about 100% (or "an order of magnitude" in some cases) compared to the old limits.
• The "Light Mediator" Surprise: This is the most exciting part. Other giant detectors (like Super-Kamiokande in Japan, which is huge and filled with water) are usually better at catching these things because they are so big. However, those big detectors have a high "energy threshold"—they need a huge splash to see anything.
  • The LZ detector is smaller but much more sensitive to tiny splashes (low energy threshold).
  • When the "slingshot" involves a light mediator, the splash is small but fast.
  • The Result: In this specific scenario, the LZ detector is actually the best in the world at catching these boosted ghosts, beating even the massive water tanks.

The "Mediator" Analogy

To understand why the results change, imagine the interaction between the Cosmic Ray and the Dark Matter:

• Heavy Mediator: Imagine the Cosmic Ray hits the Dark Matter with a sledgehammer. The force is the same no matter how fast they are moving.
• Light Mediator: Imagine they interact via a magnetic field. If they are moving slowly, the magnet is weak. If they are zooming past each other, the interaction changes completely.

The paper shows that if the interaction works like a magnet (light mediator), the LZ detector is the perfect tool to find it, whereas the giant water detectors might miss it entirely because the "splash" is too small for them to register.

Summary

The paper says: "We looked at the newest data from the LZ detector. We found that if light dark matter gets a speed boost from cosmic rays, this detector is now the most powerful tool we have to find it, especially in scenarios where the force carrying the energy is very light. We have now ruled out more possibilities than ever before, narrowing down the search for these invisible particles."

Technical Summary

Technical Summary: Cosmic Ray Electron Boosted Light Dark Matter: Implications of LZ 2025 Data

Problem Statement
Current multi-ton direct detection (DD) experiments, such as XENONnT and LUX-ZEPLIN (LZ), have placed stringent constraints on GeV-scale galactic dark matter (DM), pushing allowed cross-sections toward the "neutrino fog." However, these detectors remain largely insensitive to sub-MeV light dark matter. Non-relativistic halo DM particles with MeV-scale masses and velocities ($\sim 10^{-3}c$) fail to generate recoil energies above the keV-scale thresholds of these detectors. To address this detection gap, the "boosted dark matter" (BDM) paradigm is explored, where a subdominant population of light DM gains relativistic kinetic energy through scattering with high-energy cosmic rays (CRs). While previous works have considered BDM boosted by blazar jets, supernovae, or solar reflection, this work specifically investigates DM boosted by cosmic ray electrons (CRe) and evaluates the sensitivity of the LZ experiment using its latest 2025 data (WS2024 run).

Methodology
The authors calculate the flux of CRe-boosted DM ($\chi$) reaching Earth-based detectors and the resulting event rates in LZ.

1. Boosted Flux Calculation: The differential flux of boosted DM, $d\Phi_\chi/dT_\chi$, is derived by integrating the local interstellar CRe flux ($d\Phi_{CR}^e/dT_e$) over the DM-electron scattering cross-section. The calculation incorporates a line-of-sight integral of the Milky Way's DM density profile (assuming a Navarro–Frenk–White profile) to determine an effective diffusion parameter ($D_{eff} \approx 1$ kpc).
2. Cross-Section Models: Unlike previous studies that often assume energy-independent (constant) cross-sections, this work analyzes realistic, energy-dependent differential cross-sections arising from two specific mediator scenarios:
   • Vector Mediator: Interaction mediated by a vector boson.
   • Scalar Mediator: Interaction mediated by a scalar boson.
     The differential cross-sections include a form factor $F_{DM}(q^2)$ that accounts for the mediator mass ($m_{med}$), distinguishing between "heavy mediator" ($m_{med} \gg q$) and "light mediator" ($m_{med} \ll q$) limits.
3. Event Rate Simulation: The expected recoil spectrum in the LZ detector is calculated by convolving the boosted DM flux with the differential scattering cross-section between the boosted DM and target electrons in Xenon. The analysis incorporates:
   • Atomic binding energy effects via an effective charge function $Z_{eff}(E_R)$.
   • Detector efficiency and resolution using the 1D region-of-interest (ROI) efficiency curve and Gaussian smearing from the LZ WS2024 data release (3.3 ton$\times$year exposure).
4. Statistical Analysis: Constraints are derived by calculating the $\chi^2$ value for the $m_\chi$ vs. $\bar{\sigma}_{\chi e}$ parameter space, marginalizing to establish 2$\sigma$ confidence level (C.L.) exclusion limits.

Key Contributions and Results

• Improved Constraints on Constant Cross-Sections: Using the LZ 2025 data, the authors establish 2$\sigma$ C.L. limits on CRe-boosted DM with constant cross-sections. For a DM mass of $m_\chi \sim 0.1$ MeV, the new limits exclude cross-sections ($\bar{\sigma}_{\chi e}$) approximately one order of magnitude ($\sim O(1)$) smaller than previous XENONnT limits.
• Impact of Energy-Dependent Cross-Sections: The study demonstrates that assuming realistic energy-dependent cross-sections drastically alters the exclusion limits compared to constant cross-section assumptions.
  • Heavy Mediator Limit: In the limit where the mediator mass is large ($F_{DM} \approx 1$), the LZ 2025 constraints with energy-dependent cross-sections are an order of magnitude stronger than those derived with constant cross-sections. For vector mediators, the LZ limit is roughly a factor of 2 stronger than existing XENONnT BDM limits.
  • Scalar vs. Vector: In the low-mass region ($m_\chi \lesssim 0.1$ MeV), constraints for scalar mediators are weaker than for vector mediators due to recoil energy suppression in the scalar differential cross-section.
• Light Mediator Limit and Threshold Advantages: In the light mediator limit ($F_{DM} \propto 1/q^2$), constraints from large-volume neutrino detectors (Super-Kamiokande, IceCube) are significantly weakened due to their high energy thresholds (O(100) MeV to O(500) GeV). Conversely, LZ's low threshold ($\sim 1$ keV) allows it to place world-leading direct detection constraints on CRe-boosted DM in this specific parameter space, excluding regions previously unexplored by neutrino detectors.
• Benchmark Model Analysis: The authors present a benchmark vector mediator model (Lagrangian with couplings $g_V^\chi$ and $g_V^e$) to map constraints onto the mediator mass ($m_V$) vs. coupling ($g_V^e$) plane. For $m_\chi = 1$ MeV and $m_V \lesssim 10^{-2}$ MeV, the LZ 2025 limit on the electron coupling is approximately 6 times stronger than the XENONnT 2022 limit.

Significance
The paper claims that the LZ experiment, leveraging its low energy threshold and substantial exposure from the WS2024 run, provides the most stringent direct detection constraints for MeV-scale electrophilic dark matter boosted by cosmic ray electrons in specific parameter spaces. Specifically, the work highlights that:

1. LZ can probe regions of the mediator parameter space that are inaccessible to current neutrino detectors when considering light mediators.
2. The inclusion of realistic, energy-dependent cross-sections is crucial for accurate limit setting, often yielding significantly stronger constraints than constant cross-section approximations.
3. The study effectively utilizes the latest LZ data to push the boundaries of light dark matter searches, excluding previously unexplored regions of the mediator mass and coupling space.

The authors note that while attenuation effects in the Earth could impose upper bounds on cross-sections for heavy mediators, these were not included in the current analysis, which focuses on constraints up to $\sigma_{\chi e} \approx 10^{-29}$ cm$^2$.