⚛️ phenomenology

The LUX-ZEPLIN Event as Hyperfine Spectroscopy of Composite Dark Matter

This paper proposes that the 248 keV nuclear-recoil candidate observed by LUX-ZEPLIN can be explained as the endothermic hyperfine excitation of a single composite dark hadron within a PQ-augmented dark QCD framework, where a pseudoscalar-to-vector transition naturally produces a localized high-energy recoil peak.

Original authors: Jie Sheng, Kairui Zhang

Published 2026-09-22

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CC0 1.0

Original authors: Jie Sheng, Kairui Zhang

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ 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

Dark matter is the invisible substance that holds galaxies together, yet we have never seen a single particle of it. For decades, scientists have searched for it by building massive detectors deep underground, hoping to catch a rare collision between a dark matter particle and an ordinary atomic nucleus. The leading theory has long been that these particles bounce off nuclei like billiard balls, transferring a small amount of energy that creates a faint flash of light. However, this simple picture struggles to explain a specific, puzzling signal recently recorded by the LUX-ZEPLIN experiment. That signal was a nuclear recoil with an energy of 248 keV, a value far higher than what standard theories predict for a typical dark matter collision. If dark matter behaves like a standard particle, it should produce a flood of low-energy hits, not a solitary, high-energy spike. This discrepancy has forced physicists to reconsider the very nature of the invisible matter filling our universe.

A new study proposes a radical shift in perspective: rather than being a simple, indivisible particle, dark matter might be a composite object, much like an atom is made of a nucleus and electrons. The researchers suggest that the mysterious 248 keV signal comes from a dark matter particle that absorbs energy to change its internal state, similar to how an electron in an atom jumps to a higher energy level when hit by light. In this scenario, the dark matter particle is not just a single point but a bound system of smaller, hidden particles held together by a force similar to the one that binds protons and neutrons in our own world. The particle exists in a low-energy "ground state," but it can be excited into a slightly heavier, higher-energy state. The energy required to make this jump is exactly the amount needed to explain the high-energy recoil seen in the detector.

The authors of the paper, Jie Sheng and Kairui Zhang, construct a detailed model where this dark matter is a "dark meson," a heavy particle made of a heavy dark quark and a light dark quark. In their framework, the dark matter particle usually sits in a quiet, stable state. However, when it collides with a xenon nucleus in the detector, it can absorb enough kinetic energy to flip its internal spin configuration, transforming into a slightly heavier partner particle. This process is endothermic, meaning it consumes energy from the collision to create the mass difference between the two states. Because the collision must provide this extra energy, only the fastest-moving dark matter particles in our galaxy have enough speed to trigger the event. This requirement naturally filters out the slow particles that would create low-energy noise, leaving behind a sharp, localized peak at the high energy observed by LUX-ZEPLIN.

To make this idea work without contradicting other observations, the researchers introduce a hidden symmetry that acts as a protective shield. In many similar theories, the excited state of the dark matter would be stable and would linger in the universe, causing unwanted low-energy signals through a different type of collision. The new model solves this by including a mechanism that allows the excited state to decay rapidly into the stable ground state and a new, very light particle called a dark axion. This decay happens long before the formation of galaxies, ensuring that the universe is filled almost entirely with the stable ground-state particles. Consequently, the detector sees only the rare, high-energy events where the stable particle gets excited, while the dangerous low-energy signals from lingering excited particles are completely eliminated.

The study finds that this explanation fits the data remarkably well if the dark matter particle has a mass between 500 and 1,000 times that of a proton, and if the energy gap between its two states is roughly 300 to 400 keV. These specific values align with the 248 keV signal recorded by the experiment. The model also predicts that the dark matter interacts with ordinary matter through a force carrier called a dark photon, which mixes slightly with the electromagnetic force. This connection offers a way to test the theory: if the model is correct, experiments searching for these dark photons should find them, and future measurements of the recoil energy spectrum should reveal the specific shape predicted by this internal excitation mechanism.

This interpretation reframes the search for dark matter from looking for a simple collision to performing a form of spectroscopy on invisible atoms. Just as astronomers analyze the light from stars to determine their chemical composition, this approach suggests we can learn about the internal structure of dark matter by studying the specific energy it absorbs during a collision. While the idea remains a hypothesis that requires further verification, it offers a coherent and elegant solution to a stubborn anomaly. It suggests that the dark sector is not a barren landscape of simple particles, but a complex world with its own rich internal structure, waiting to be revealed by the precise measurements of the next generation of experiments.

Technical Summary: The LUX-ZEPLIN Event as Hyperfine Spectroscopy of Composite Dark Matter

Problem Statement
The LUX-ZEPLIN (LZ) collaboration recently reported a nuclear-recoil candidate event with an energy of $E_R = 248 \pm 23 \text{ (stat)} \pm 23 \text{ (sys)}$ keV. While the local significance is $3.4\sigma$ (reduced to $2.6\sigma$ globally), this event poses a challenge for standard Weakly Interacting Massive Particle (WIMP) models. Conventional elastic scattering predicts a spectrum favoring low-energy recoils, which is suppressed at high energies by the galactic halo velocity distribution and nuclear form factors.

Endothermic scattering ( $\chi_1 N \to \chi_2 N$ with mass splitting $\Delta > 0$ ) has emerged as a leading interpretation, as it selects the high-velocity tail of the halo and confines recoils to a narrow, high-energy interval. However, existing models face significant hurdles:

Higgsino DM: While offering a near-degenerate splitting, thermal Higgsino interpretations struggle with naturalness and collider constraints. Furthermore, inelastic scattering in the Sun can lead to annihilation into neutrinos, which is strongly constrained by IceCube and Super-Kamiokande.
Composite Inelastic Dark Matter (CiDM): Standard CiDM models, where Dark Matter (DM) is a heavy-light meson with a pseudoscalar ground state ( $\pi_d$ $π_{d}$ ) and a vector excitation ( $\rho_d$ $ρ_{d}$ ), face issues with:
- Stability: Ensuring the ground state $\pi_d$ is stable against decay into lighter dark hadrons.
- Elastic Scattering: A generic dark vacuum angle ( $\bar{\theta}_d$ ) violates parity and induces unacceptably large elastic scattering signals.
- Exothermic Background: A cosmologically long-lived population of excited states ( $\rho_d$ ) would produce a low-energy exothermic recoil background ( $\rho_d N \to \pi_d N$ ) that is not observed.

Methodology and Model Construction
The authors propose a solution within a Peccei-Quinn (PQ)-augmented dark QCD framework. The model introduces an $SU(N)_d \times U(1)_{A'}$ gauge theory with four left-handed Weyl fermions ( $\chi_{h,l}, \chi^c_{h,l}$ ) and a global $U(1)_{PQ}$ symmetry.

Mass Hierarchy and Symmetry: The PQ symmetry forbids bare flavor-mixing masses and light-quark Yukawa couplings, generating a heavy-light quark mass hierarchy ( $m_h \gg m_l$ ) via interactions with a dark Higgs $\Phi$ and a singlet scalar $S$ . This setup yields accidental flavor symmetries ( $U(1)_h \times U(1)_l$ ) that stabilize the lightest heavy-light meson, the pseudoscalar $\pi_d$ , and protect primordial flavor asymmetries.
Hyperfine Splitting: The DM candidate is the $\pi_d$ meson. The excited state is the vector meson $\rho_d$ . Due to heavy-quark spin symmetry, these states are nearly degenerate, with a splitting $\Delta \equiv m_{\rho_d} - m_{\pi_d}$ generated by subleading hyperfine interactions, scaling as $\Delta \sim \Lambda_d^2 / m_h$ .
Suppression of Elastic Scattering: The model includes a dark axion $a_d$ coupled to the $SU(N)_d$ and $U(1)_{A'}$ sectors. The dark axion dynamically relaxes the physical vacuum angle $\bar{\theta}_d$ to a parity-conserving minimum, thereby suppressing the dangerous elastic scattering channel that would otherwise arise from a generic $\bar{\theta}_d$ .
Depletion of Excited States: The same PQ transition current allows for the decay $\rho_d \to \pi_d a_d$ . The authors calculate that for appropriate parameters, this decay depletes the primordial $\rho_d$ population before structure formation, eliminating the low-energy exothermic recoil background.
Interaction with SM: The dark photon $A'$ acquires mass via the breaking of $U(1)_{A'}$ and couples to the Standard Model via kinetic mixing ( $\epsilon$ ). This mediates the transition $\pi_d N \to \rho_d N$ .

Key Results
The authors analyze the parameter space required to explain the LZ event as an endothermic hyperfine excitation:

Kinematics: For a DM mass $m_{\pi_d} \sim \text{sub-TeV to TeV}$ and a splitting $\Delta \sim 300 \text{ keV}$ , the endothermic threshold selects a narrow range of high-velocity halo particles. This naturally produces a localized recoil peak near the observed 248 keV.
Event Rate: By normalizing the differential recoil rate to yield exactly one detected event in the LZ window ( $248 \pm 1\sigma_{stat} \pm 1\sigma_{sys}$ ), the authors identify viable benchmark points (e.g., $m_{\pi_d} = 750 \text{ GeV}, \Delta = 360 \text{ keV}$ ).
Spectral Features: The predicted recoil spectrum exhibits a sharp peak near the threshold energy. The authors note that for heavier DM masses, a secondary high-energy tail appears due to the nuclear charge form factor diffraction minimum, but this can be constrained by the absence of events in the LZ high-energy sideband.
Constraints:
- Dark Photon Limits: The required kinetic mixing $\epsilon$ is checked against experimental bounds (specifically for $m_{A'} = 0.2 \text{ GeV}$ , where $A' \to e^+e^-$ ). The model requires non-perturbative couplings or large hadronic coefficients only if the mixing approaches the current exclusion limits.
- Exothermic Background: The model successfully suppresses the exothermic contribution via the $\rho_d \to \pi_d a_d$ decay, avoiding the low-energy excess that plagues other CiDM interpretations.
- Cosmology: The parameter space consistent with the LZ event is shown to satisfy cosmological constraints, including the domain-wall problem (solved by $N_{DW}=1$ ) and relic abundance.

Significance and Claims
The paper claims to offer a specific, testable interpretation of the LZ high-recoil event as "hyperfine spectroscopy of composite dark matter."

Unified Origin of Splitting: Unlike models requiring two distinct dark-sector particles, this framework attributes the required energy splitting to the internal hyperfine structure of a single composite dark hadron (analogous to $B-B^*$ or $\eta_c-J/\psi$ in SM QCD).
Resolution of Tensions: The introduction of the dark PQ symmetry simultaneously addresses three critical challenges: it stabilizes the DM ground state, dynamically suppresses parity-violating elastic scattering, and removes the dangerous low-energy exothermic background via axion-mediated decay.
Testability: The authors emphasize that this interpretation is subject to complementary tests. Specifically, searches for the kinetically mixed dark photon (decaying to $e^+e^-$ ) and future high-precision measurements of the nuclear recoil spectrum can validate or rule out this "hyperfine spectroscopy" hypothesis.

The authors conclude that sub-TeV scale dark matter with a hyperfine splitting of a few hundred keV can naturally yield an order-one event rate in the LZ high-recoil window, providing a coherent alternative to standard WIMP scenarios.

Technical Summary: The LUX-ZEPLIN Event as Hyperfine Spectroscopy of Composite Dark Matter

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