⚛️ phenomenology

ALP-mediated inelastic dark matter and the LUX-ZEPLIN high-recoil candidate event LZ230616

This paper proposes that the high-energy candidate event LZ230616 observed by the LUX-ZEPLIN experiment could be explained by inelastic scattering between Majorana dark matter states mediated by an axion-like particle coupled to gluons, identifying specific mass and coupling parameters that reproduce the observed recoil energy while highlighting the model's sensitivity to halo velocity constraints.

Original authors: Guan-Wen Yuan, Bo Zhang, Wen-Yu Cao, Lei Feng, Ruizhi Yang

Published 2026-09-09

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CC BY 4.0

Original authors: Guan-Wen Yuan, Bo Zhang, Wen-Yu Cao, Lei Feng, Ruizhi Yang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep in the silence of the universe, a mystery persists: what is dark matter? It is the invisible substance that holds galaxies together, yet it refuses to reveal itself to our telescopes. For decades, scientists have built massive detectors deep underground, shielding them from cosmic noise, hoping to catch a single, faint whisper of a dark matter particle bumping into an ordinary atom. The most sensitive of these instruments is the LUX-ZEPLIN experiment, a tank of liquid xenon buried in a mine in South Dakota. Recently, this detector recorded a single, unusual flash of light. It was a signal of a particle striking a xenon nucleus with an energy far higher than scientists typically expect to see. While this single event does not prove the existence of dark matter, its surprising energy level has sparked a new line of inquiry, prompting researchers to ask if this flash could be the signature of a more complex, hidden world than previously imagined.

A team of physicists has now explored a specific possibility to explain this high-energy flash. They propose that the dark matter particle is not a single, static entity, but rather part of a two-state system, similar to how an atom can exist in different energy levels. In this scenario, a lighter dark matter particle travels through space and collides with a xenon nucleus. However, instead of simply bouncing off, the collision forces the particle to jump into a heavier, excited state. This transition requires a significant amount of energy, much like a ball rolling up a steep hill needs a strong push to reach the top. Because of this energy cost, low-speed collisions simply cannot happen; they lack the force needed to trigger the jump. This mechanism naturally filters out the weak, low-energy signals that usually clutter these detectors, leaving only the rare, high-energy impacts.

The researchers combined this idea of a "heavy jump" with another concept involving a hypothetical particle called an axion-like particle. This particle acts as a messenger, carrying the force between the dark matter and the ordinary matter in the detector. Crucially, the strength of this interaction changes depending on how fast the particles are moving and how hard they hit. When the team simulated this specific combination of a two-state dark matter particle and a momentum-dependent messenger, they found that it could reproduce the exact energy of the mysterious flash recorded by LUX-ZEPLIN. The simulation showed that if the dark matter particle has a mass of about 350 gigaelectronvolts and the energy gap to its heavier state is roughly 330 kiloelectronvolts, the resulting collision would create a sharp, concentrated spike of energy right where the detector saw the event.

However, the researchers are careful to note that this explanation comes with significant caveats. To make this specific scenario work, the interaction between the dark matter and the messenger particle would need to be unusually strong. Furthermore, the model relies heavily on the existence of the fastest-moving dark matter particles in our galaxy. If the speed limit of these particles in our cosmic neighborhood is slightly different than assumed, the entire explanation could fall apart. The team also identified other possibilities that fit the data slightly less perfectly but require much weaker interactions and are less sensitive to the speed of the particles. These alternative scenarios involve much heavier dark matter particles, in the range of one to five teraelectronvolts, which would produce a broader, less sharp signal but still match the observed energy.

Ultimately, this study does not claim to have solved the mystery of dark matter. Instead, it maps out the specific conditions required for a particular theory to explain the LUX-Zeplin event. The authors emphasize that a single flash cannot confirm a new particle; it merely suggests a path forward. To know if this theory is true, future work must check if the required particle properties align with what we know about the universe's history and with other experiments searching for similar particles in laboratories. Until then, the high-energy flash remains a tantalizing clue, pointing toward a dark sector that may be far more dynamic and complex than the simple, static particles we have long imagined.

Technical Summary: ALP-mediated Inelastic Dark Matter and the LUX-ZEPLIN High-Recoil Candidate Event LZ230616

Problem Statement
The LUX-ZEPLIN (LZ) Collaboration recently reported a candidate nuclear recoil event, LZ230616, with a reconstructed energy of $E_R = 248 \pm 23_{\text{stat}} \pm 23_{\text{sys}}$ keV. This event occurs in a low-background region but possesses an unusually high recoil energy ( $Q \approx 246$ MeV), exceeding the typical kinematic reach of standard elastic dark matter (DM) scattering, where phase space and nuclear form factors usually suppress high-energy recoils. The authors investigate whether this event can be interpreted as inelastic scattering between two Majorana DM states mediated by an axion-like particle (ALP) coupled to gluons.

Methodology
The study employs a two-state Majorana dark sector model where the incident DM particle $\chi_1$ scatters into a heavier state $\chi_2$ ( $\chi_1 N \to \chi_2 N$ ) with a positive mass splitting $\delta = m_{\chi_2} - m_{\chi_1} > 0$ . The interaction is mediated by an ALP ( $a$ ) via a gluonic portal.

Theoretical Framework: The Lagrangian includes an off-diagonal derivative coupling between the ALP and the DM states ( $\lambda_\chi$ ) and a gluonic coupling ( $\lambda_G$ ) via the operator $a G \tilde{G}$ . The authors treat the dark-sector and gluonic couplings independently.
Nuclear Physics: Unlike simplified models, the calculation retains full momentum-dependent nucleon form factors and nuclear responses. The pseudoscalar gluonic matrix element is modeled including pion and eta pole contributions. The scattering is dominated by the non-relativistic operator $\mathcal{O}_6 \propto (\vec{S}_\chi \cdot \vec{q})(\vec{S}_N \cdot \vec{q})$ .
Kinematics and Rate Calculation: The minimum incident velocity required to produce a recoil $E_R$ is derived, showing that positive $\delta$ suppresses low-energy recoils and shifts the spectrum to higher energies. The event rate is calculated using the Standard Halo Model (SHM) with specific parameters ( $v_0 = 238$ km/s, $v_{\text{esc}} = 544$ km/s, $\rho_\chi = 0.3$ GeV/cm $^3$ ), averaging the inverse-speed integral over a full year to account for Earth's motion.
Statistical Analysis: The authors construct an approximate single-event likelihood based on the recoil energy spectrum. They define a profile likelihood ratio $\Delta q_{\text{LZ}}$ to assess spectral compatibility between the model prediction and the observed event, conditional on a signal interpretation. The coupling product $|\lambda_\chi \lambda_G|$ is normalized to yield exactly one expected event in the region of interest (ROI).

Key Contributions and Results
The paper identifies specific regions in the parameter space ( $m_\chi, \delta, m_a$ ) that can reproduce the spectral shape of the LZ230616 event:

Spectral Shape vs. Coupling: For a mediator mass $m_a = 0.3$ GeV, the "best-fit" spectral shape occurs at $m_\chi \approx 350$ GeV and $\delta \approx 330$ keV. This configuration produces a narrow peak near 248 keV because only DM particles with velocities near the Galactic escape speed can overcome the kinematic threshold. At this specific point, $(m_\chi, \delta) = (350.2 \text{ GeV}, 330 \text{ keV})$ , producing one expected event requires a coupling product of $|\lambda_\chi \lambda_G| \simeq 1.51$ GeV $^{-2}$ . This configuration is highly sensitive to the precise value of the halo speed cutoff. (Note: While Table I lists a value of 0.21 GeV $^{-2}$ for the best fit, the text explicitly states the requirement is $\simeq 1.51$ GeV $^{-2}$ for the one-event normalization at this benchmark).
Alternative Benchmarks: The authors demonstrate that relaxing the spectral agreement criterion (allowing $\Delta q_{\text{LZ}} \le 1$ or $\le 2$ ) opens up alternative solutions at the TeV scale (e.g., $m_\chi \approx 1.25$ TeV, $\delta \approx 320$ keV). These solutions yield broader spectra but require coupling products smaller by more than three orders of magnitude ( $|\lambda_\chi \lambda_G| \approx 2.30 \times 10^{-4}$ GeV $^{-2}$ ) and are less sensitive to the halo velocity endpoint.
Degeneracy: The analysis reveals a degeneracy between the DM mass and the mass splitting when the coupling is treated as a free normalization parameter. While the recoil energy constrains the relationship between $m_\chi$ and $\delta$ , the event count alone cannot isolate a unique point in the $(m_\chi, \delta)$ plane without additional constraints.

Significance and Claims
The authors emphasize that their analysis establishes the kinematic and coupling requirements for interpreting LZ230616 within this specific ALP-mediated inelastic framework, but they do not claim to have confirmed a dark matter origin.

Limitations: The study uses an approximate likelihood that does not reproduce the official LZ detector likelihood or define statistically calibrated confidence regions. The interpretation assumes the present-day dominance of the lighter state $\chi_1$ , leaving the consistency of this assumption with the thermal relic abundance of the two-state system for future work.
Future Tests: The paper concludes that a single candidate event cannot establish a unique or fully viable DM solution. A self-consistent interpretation requires:
1. A calculation of the thermal relic abundance to constrain $\lambda_\chi$ .
2. Independent laboratory tests (colliders, rare meson decays) to constrain $\lambda_G$ .
3. Future high-energy recoil searches and indirect detection experiments to probe the heavier state and the specific kinematic features identified.

In summary, the paper provides a rigorous kinematic and phenomenological mapping of how a gluonic ALP portal could explain the LZ high-recoil candidate, highlighting the trade-off between spectral sharpness and coupling strength, while explicitly deferring the validation of the model's cosmological and particle-physics consistency to subsequent studies.

Technical Summary: ALP-mediated Inelastic Dark Matter and the LUX-ZEPLIN High-Recoil Candidate Event LZ230616

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