⚛️ phenomenology

Decaying Dark Matter for the Exothermic Recoil at LZ

This paper proposes that the single nuclear-recoil candidate observed by the LUX-ZEPLIN (LZ) experiment at approximately 248 keV originates from the exothermic downscattering of a GeV-scale dark matter state, which is continuously replenished by the decay of a long-lived parent particle, thereby predicting a corresponding signal in argon detectors and constraining the model's photon coupling parameters.

Original authors: Yongsoo Jho, Sanghwan Kim, Seong Chan Park

Published 2026-09-30

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CC BY 4.0

Original authors: Yongsoo Jho, Sanghwan Kim, Seong Chan Park

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 beneath the surface of the Earth, in a tank of liquid xenon kept colder than outer space, scientists are listening for a whisper. This is the LUX-ZEPLIN experiment, a massive detector designed to catch the faintest possible touch from dark matter, the invisible substance that makes up most of the mass in our universe. For years, these detectors have searched for dark matter particles bumping into atomic nuclei, expecting to see a steady stream of tiny energy flashes. Recently, however, the experiment reported something unusual: a single, isolated flash of energy at a very specific level, roughly 248 kiloelectronvolts. It was a solitary event, not enough to declare a discovery, but its high energy and its isolation from other background noise sparked a new line of thinking. Instead of dismissing it as a fluke, a team of researchers asked what kind of physics could produce such a precise, high-energy signal without a trail of lower-energy noise leading up to it.

The researchers propose that this single event is the signature of a dark matter particle that is not a single, static entity, but part of a family with different internal states. Imagine a dark matter particle that exists in a slightly excited, heavier state. As it travels through the galaxy and encounters an atomic nucleus in the detector, it does not just bounce off; it drops down to a lower, lighter state. In doing so, it releases a packet of energy that comes from the difference in mass between the two states, rather than from the speed of the particle itself. This process, known as exothermic scattering, acts like a tiny internal battery discharging at the moment of impact. Because the energy comes from this internal mass difference, the resulting flash of light is incredibly sharp and narrow, appearing at a fixed energy level regardless of how fast the particle was moving when it arrived. This explains why the detector saw a single, clean line at 248 keV without the messy, spread-out tail of lower energies that usually accompanies such collisions.

For this scenario to work, the researchers suggest that the universe is currently filled with these excited particles because they are being constantly replenished. They posit that a much heavier, long-lived "parent" particle, which has been decaying slowly over billions of years, is continuously creating new pairs of these excited particles. This mechanism ensures that there is always a fresh supply of the excited state ready to interact with the detector, even if the original population from the birth of the universe has long since vanished. The energy released in the drop from the excited state to the ground state is just right to match the 248 keV signal seen in the xenon tank, provided the particles have a mass in the range of a few hundred million electronvolts to several billion electronvolts.

The beauty of this explanation lies in its predictability. If this specific type of dark matter interaction is real, it should produce a similar signal in other types of detectors, but at a different energy level determined by the mass of the target atoms. The researchers calculated that if the same particles were to strike an argon detector, they would produce a sharp line of energy between 700 and 810 keV, depending on the exact mass of the dark matter particle. Similarly, a germanium detector would see a line between 420 and 450 keV. This distinct pattern, where the energy of the signal shifts in a precise way based on the material used, offers a clear way to test the idea. Unlike other theories that rely on the speed of the particles or complex cosmic histories, this model ties the signal directly to the internal structure of the dark matter itself.

However, this scenario comes with a cost that must be checked against the rest of the universe. If these particles can drop from a high state to a low state by interacting with nuclei, they should also be able to do so by emitting light, specifically pairs of photons. The researchers calculated how often this light emission should happen in our galaxy based on the single event seen in the xenon tank. They found that for lighter particles, the rate of this light emission would be so high that it would likely have been seen by telescopes scanning the sky for gamma rays. To avoid contradicting existing observations, the theory requires that the particles be relatively heavy, around 10 billion electronvolts, or that the mechanism for emitting light is heavily suppressed. This creates a tight constraint: the model works, but only if the dark matter particles are heavy enough and their ability to glow is carefully tuned.

Ultimately, this paper does not claim to have solved the mystery of dark matter. A single event is not a proof, and the model remains a suggestion of what could be happening. Instead, it offers a concrete, testable hypothesis that turns a puzzling anomaly into a roadmap. It suggests that if we look at the right energy levels in argon or germanium detectors, we might find the same sharp line that appeared in the xenon tank. If those future detectors see the predicted signals, it would confirm that dark matter has a rich internal structure and that the universe is filled with particles that are slowly decaying and reshaping themselves. If they do not, the idea can be discarded. Until then, the single flash at 248 keV stands as a quiet, precise invitation to look deeper into the hidden architecture of the cosmos.

Technical Summary: Decaying Dark Matter for the Exothermic Recoil at LZ

Problem Statement
The LUX-ZEPLIN (LZ) experiment reported a single nuclear-recoil candidate event at an energy of $E_R = 248 \pm 23(\text{stat}) \pm 23(\text{sys})$ keV. While a single event does not constitute a discovery, its high energy and the absence of a corresponding low-energy excess challenge standard background-only hypotheses, which show a global tension of $2.6\sigma$ . Existing interpretations often rely on endothermic scattering, boosted dark matter, or heavy exothermic dark matter (DM). However, these models face specific kinematic or phenomenological constraints: endothermic scattering requires high-velocity tails or boosted fluxes to reach 248 keV; heavy exothermic models typically predict a target-independent recoil energy ratio ( $E_{Ar}/E_{Xe} \to 1$ ); and standard boosted scenarios require a monochromatic flux tuned precisely to a kinematic threshold. The authors propose a mechanism that generates a narrow, high-energy recoil line without relying on halo velocity tails or boosted incident momentum.

Methodology and Model Framework
The authors propose a "decay regeneration" scenario involving a GeV-scale excited dark state $\chi_1$ and a long-lived parent $\chi_0$ . The mechanism operates via two sequential processes:

Regeneration: A long-lived parent $\chi_0$ decays slowly in the galactic halo via $\chi_0 \to \chi_1 \chi_1$ . This continuously replenishes the population of $\chi_1$ , assuming the primordial abundance of $\chi_1$ is negligible.
Exothermic Scattering: The $\chi_1$ state downscatters off xenon nuclei ( $A$ ) via $\chi_1 A \to \chi_2 A$ , where $\chi_2$ is a lighter dark state. The mass splitting $\delta = m_{\chi_1} - m_{\chi_2}$ is on the MeV scale.

The recoil energy is supplied primarily by the mass splitting $\delta$ rather than the incident momentum. The kinematics are derived for a parent at rest, where the daughter $\chi_1$ receives a small "kick" velocity $v_{kick} \simeq \sqrt{\epsilon/m_{\chi_1}}$ (with $\epsilon = m_{\chi_0} - 2m_{\chi_1}$ ). This small kick ensures the recoil line remains narrow, as the recoil energy is nearly independent of the halo velocity distribution.

The authors utilize a scalar mediator $\phi$ coupling to nucleons and the dark states. They analyze the nuclear response using the Helm form factor with natural xenon isotopes, noting that the 248 keV candidate lies just below the second Helm zero, making the event rate sensitive to nuclear model variations.

Key Contributions and Results

Kinematic Constraints and Target Dependence:
The model requires $m_{\chi_1} \gtrsim 247$ MeV to produce a 248 keV recoil on xenon. Unlike heavy exothermic DM models, this GeV-scale scenario predicts a strong target dependence. The recoil energy in a different target $B$ is given by $E_B^{(0)} / E_A^{(0)} = (M_A + m_{\chi_1}) / (M_B + m_{\chi_1})$ .
- For $m_{\chi_1} = 0.3$ GeV, the model predicts an argon line at $\sim 810$ keV.
- For $m_{\chi_1} = 10$ GeV, the argon line shifts to $\sim 700$ keV.
- This contrasts with heavy DM models ( $E_{Ar}/E_{Xe} \approx 1$ ) and endothermic boosted models (which generally do not produce a line in argon at the same relative energy).
Spectral Characteristics:
The recoil spectrum is a narrow line with no low-energy tail. The width is determined by the non-relativistic motion of $\chi_1$ and the small kick from the parent decay. For the benchmark $m_{\chi_1} = 10$ GeV, the line width is $\sigma_R \approx 12$ keV; for $m_{\chi_1} = 0.3$ GeV, it is $\approx 4$ keV. The position is robust against nuclear response uncertainties, shifting by less than a few keV even when Helm parameters are varied, though the event rate can change by an order of magnitude.
Radiative Decay Constraints:
The same mass splitting $\delta$ that drives the scattering also drives the radiative decay $\chi_1 \to \chi_2 \gamma \gamma$ (via an off-shell mediator). Normalizing the model to the single LZ event fixes the local emissivity of this decay.
- The authors estimate the required photon coupling $r_\gamma$ (relative to a proton-loop reference).
- For sub-GeV daughters ( $m_{\chi_1} \sim 0.3$ GeV), the coupling must be highly suppressed ( $r_\gamma \lesssim 10^{-9}$ ).
- For GeV-scale daughters ( $m_{\chi_1} \sim 10$ GeV), the constraint is significantly looser ( $r_\gamma \lesssim 10^{-2}$ ).
- This suggests that a 10 GeV benchmark is more natural as it requires less fine-tuning of the photon coupling to avoid conflict with $\gamma$ -ray observations.
Explicit Benchmarks:
The paper provides two benchmarks (L: 0.3 GeV, H: 10 GeV). For the 10 GeV case, an explicit effective theory is constructed with a mediator mass $m_\phi = 2$ GeV and specific couplings that reproduce the LZ event rate while satisfying the radiative decay constraints.

Significance and Claims
The paper argues that the LZ candidate is "naturally explained" by the exothermic downscattering of a GeV-scale excited state regenerated by the decay of a long-lived parent. The primary significance of this proposal lies in its distinct phenomenological predictions:

Narrow Line: It produces a narrow recoil line without requiring a velocity tail or boosted flux.
Target Ratio: It predicts a specific, mass-dependent ratio of recoil energies between xenon, argon, and germanium ( $E_{Ar}/E_{Xe} \approx 2.8\text{--}3.3$ ), which serves as a sharp test to distinguish this model from other exothermic or boosted scenarios.
Radiative Constraints: It links the direct detection rate to the local emissivity of $\chi_1 \to \chi_2 \gamma \gamma$ , providing a testable constraint on the mediator's photon coupling.

The authors conclude that while a single event cannot establish a signal, this scenario offers a coherent explanation that makes sharp, testable predictions for future data from argon-based detectors (e.g., DarkSide-20k) and germanium experiments, as well as for $\gamma$ -ray observations. They explicitly state that the model does not rely on a primordial excited population but rather on continuous regeneration, allowing for larger mass splittings than models requiring the excited state to survive a Hubble time.

Technical Summary: Decaying Dark Matter for the Exothermic Recoil at LZ

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