⚛️ phenomenology

Sub-MeV gamma-ray lines from an endothermic interpretation of the LZ recoil candidate

This paper proposes that if the recent high-energy nuclear-recoil candidate observed by LUX-ZEPLIN (LZ) is caused by endothermic dark matter upscattering, the same mass splitting would produce a sub-MeV gamma-ray line detectable by the upcoming COSI mission, thereby offering a critical test for this dark matter interpretation.

Original authors: Maíra Dutra, Jacinto P. Neto, Clarissa Siqueira

Published 2026-09-29

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CC BY 4.0

Original authors: Maíra Dutra, Jacinto P. Neto, Clarissa Siqueira

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 quiet dark of space, invisible particles known as dark matter are thought to drift through the universe, making up most of its mass. For decades, scientists have tried to catch these elusive particles by building massive, ultra-sensitive detectors deep underground, hoping to see a tiny flash of light when a dark matter particle bumps into an atom. Recently, a major experiment called LUX-ZEPLIN, located in a mine in South Dakota, reported a single, puzzling event. A heavy atom inside the detector seemed to have been knocked backward with a specific, high-energy kick that standard physics cannot easily explain. This single signal has sparked a flurry of theories, but one idea stands out for its ability to turn a mystery into a testable prediction: what if the dark matter particle is not a single, static object, but rather a two-part system that can change its state?

The researchers behind this new study, Maíra Dutra, Jacinto P. Neto, and Clarissa Siqueira, explored a specific version of this idea called endothermic dark matter. In this scenario, the dark matter particle exists in two forms: a lighter, stable version and a heavier, excited version. Normally, the heavier version is rare or non-existent in our galaxy. However, when a lighter dark matter particle smashes into an atomic nucleus in a detector, it can absorb some of the impact energy to transform into the heavier version. This process requires a specific amount of energy to happen, which explains why the LUX-ZEPLIN detector saw a recoil at such a high energy level. The energy needed for this transformation is determined by the difference in mass between the two dark matter states, a gap that the researchers calculated to be around 350 thousand electron volts.

While this theory offers a neat explanation for the underground signal, the team realized it also predicts something that should be visible from space. If the heavy dark matter particle is created in the galaxy, it cannot stay heavy forever; it must eventually decay back into its lighter form. When it does this, it releases the extra energy it absorbed, but instead of a nuclear kick, it emits a single, high-energy photon, or a gamma ray. Because the energy of this photon is directly tied to the mass difference between the two dark matter states, the researchers calculated that this decay would produce a very specific line of gamma rays with an energy of about 350 thousand electron volts. This is a sub-Mega-electron-volt signal, a range of energy that is difficult to observe but perfectly suited for a new space telescope called the Compton Spectrometer and Imager, or COSI, which is scheduled to launch in 2027.

The authors did not simply guess that this signal exists; they built a detailed model of how these particles would behave in our galaxy to see if the numbers add up. They started with the assumption that the dark matter in our galaxy is mostly in the lighter state, but that collisions between these lighter particles can occasionally create the heavier ones. They calculated that these collisions happen often enough in the dense center of the Milky Way to create a steady stream of heavy particles. These heavy particles then decay almost instantly, releasing the gamma rays. By simulating this process, the team found that the brightness of this gamma-ray line would be strong enough to be detected by the upcoming COSI mission, provided the dark matter behaves exactly as their model suggests.

This work is significant because it connects a single, ambiguous event in a mine to a clear, observable signal in the sky. The paper does not claim to have proven that dark matter exists or that the LUX-ZEPLIN event was definitely caused by it; the underground signal remains just one candidate event. Instead, the study shows that if that candidate event is indeed a sign of this specific type of dark matter, then the same physics must also be creating a gamma-ray line that we can look for. The researchers carefully checked their calculations against existing data from other telescopes and found that their predicted signal does not contradict current observations. They also showed that the strength of the signal depends on the speed of the dark matter particles in our galaxy, meaning that future measurements could either confirm their theory or rule it out.

The beauty of this proposal lies in its simplicity and its testability. It takes a complex idea about invisible particles changing their internal structure and translates it into a concrete search strategy. If the COSI telescope, once in orbit, scans the center of the galaxy and finds a sharp spike in gamma-ray energy at the precise level predicted, it would provide powerful, independent evidence supporting the endothermic dark matter theory. If the telescope looks and finds nothing, it would suggest that the LUX-ZEPLIN event was likely a fluke or caused by something else entirely. In either case, the paper provides a clear path forward, turning a moment of confusion in a dark mine into a focused search for light in the stars.

Technical Summary: Sub-MeV Gamma-Ray Lines from an Endothermic Interpretation of the LZ Recoil Candidate

Problem and Motivation
The LUX-ZEPLIN (LZ) collaboration recently reported a high-energy nuclear-recoil candidate event with a reconstructed energy of $E_R = 248 \pm 23 \text{ (stat)} \pm 23 \text{ (sys)}$ keV. While this event does not yet constitute a confirmed dark matter (DM) signal, it has motivated various theoretical interpretations, including endothermic inelastic DM. In this scenario, a lighter DM state ( $\chi_1$ ) scatters off a nucleus to produce a heavier state ( $\chi_2$ ), requiring the incoming DM particle to supply a mass splitting $\delta \equiv m_2 - m_1$ . This mechanism naturally shifts the recoil spectrum to higher energies, offering a potential explanation for the LZ candidate.

However, cosmological constraints and the requirement to avoid exothermic scattering limits often necessitate the depletion of the primordial heavier state ( $\chi_2$ ). The authors investigate whether this same mass splitting $\delta$ , which governs the nuclear recoil, also produces a distinct astrophysical signature: a sub-MeV gamma-ray line resulting from the radiative decay $\chi_2 \to \chi_1 \gamma$ .

Methodology
The authors propose a pseudo-Dirac dark sector model where:

Interaction: A kinetically mixed dark photon ( $A'$ ) mediates endothermic nuclear scattering ( $\chi_1 N \to \chi_2 N$ ) and DM self-interactions.
Relic Abundance: The relic density is set by annihilations into dark photons ( $\chi_i \chi_i \to A' A'$ ), calculated using micrOMEGAs considering coannihilations.
Depletion and Regeneration: To satisfy cosmological constraints, an electromagnetic transition dipole moment ( $\mu_{12}$ ) is introduced to allow the rapid radiative decay of the primordial $\chi_2$ population ( $\chi_2 \to \chi_1 \gamma$ ). Crucially, the same vector interaction that mediates scattering allows for the regeneration of $\chi_2$ in the Galactic halo via upscattering ( $\chi_1 \chi_1 \to \chi_2 \chi_2$ ).
Signal Calculation: The regenerated $\chi_2$ states promptly decay, producing a monochromatic gamma-ray line with energy $E_\gamma \simeq \delta$ . The authors calculate the integrated gamma-ray flux within $10^\circ$ of the Galactic Center (GC) using a Navarro-Frenk-White (NFW) halo profile and a truncated Maxwellian velocity distribution. They solve the coupled-channel scattering problem non-perturbatively to determine the excitation rate, as the perturbative treatment is insufficient for their benchmark parameters.

Key Contributions

Linking Recoil and Gamma-Ray Signatures: The paper establishes a direct theoretical link between the mass splitting required to explain the LZ high-energy recoil and the energy of a potential gamma-ray line ( $E_\gamma \simeq \delta$ ).
Benchmark Model: The authors define a benchmark model with $m_1 = 1$ TeV, $m_{A'} = 1$ GeV, $\delta = 350$ keV, and $\alpha_D = 0.03$ . This model satisfies the observed thermal relic abundance ( $\Omega_\chi h^2 \simeq 0.12$ ) and is consistent with the LZ recoil energy diagnostic.
Feasibility of Detection: They demonstrate that for this benchmark, the predicted gamma-ray line flux falls within the projected sensitivity of the Compton Spectrometer and Imager (COSI), a mission scheduled for launch in 2027.

Results

Flux Predictions: For the benchmark parameters, the integrated line flux within $10^\circ$ of the GC is calculated. The results show that the flux depends on the halo velocity scale ( $v_0$ ) and the mass splitting.
Constraints and Projections:
- Existing limits from INTEGRAL/SPI (specifically the NFW-only 95% C.L. limit) constrain the parameter space but do not rule out the benchmark.
- The projected 3 $\sigma$ sensitivity of COSI over a two-year exposure is shown to be capable of testing the benchmark predictions selected by the LZ energy diagnostic.
Parameter Dependence: The study varies the DM mass ( $m_1$ ) and mediator mass ( $m_{A'}$ ). It finds that increasing the mediator mass (e.g., to 2–3 GeV) can suppress the gamma-ray flux below the COSI projection while maintaining consistency with the LZ recoil interpretation and the correct relic abundance.
Decay Location: The authors confirm that for their assumed lifetime ( $\tau_\gamma \sim 10^8$ s), the decay occurs effectively locally within the halo, and the probability of a decay occurring inside the LZ detector (following an upscattering event) is negligible ( $P_{dec} \sim 10^{-14}$ ), preserving the isolated-recoil signature.

Significance and Claims
The paper claims that if the LZ candidate event is indeed a dark matter signal interpreted via endothermic inelastic scattering, it motivates dedicated searches for sub-MeV gamma-ray lines. The authors posit that the COSI mission provides a timely and complementary opportunity to test this specific interpretation. The confirmation of the LZ event as a DM signal would therefore drive a joint analysis of nuclear recoil data and sub-MeV gamma-ray line searches, with the mass splitting $\delta$ serving as the unifying parameter connecting the two observables. The work does not claim to have detected a signal but rather outlines a testable hypothesis connecting a specific experimental anomaly to a future astrophysical observation.

Technical Summary: Sub-MeV Gamma-Ray Lines from an Endothermic Interpretation of the LZ Recoil Candidate

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