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Hadrophilic inelastic freeze-in dark matter in $q_1-q_2$ gauge extension and the high energy LZ event

This paper proposes a hadrophilic $q_1-q_2$ gauge extension model featuring exothermic freeze-in dark matter mediated by a $Z'$ boson to simultaneously explain the recent high-energy LZ event and satisfy indirect detection constraints while evading flavor mixing limits.

Original authors: Xiao-Gang He, Xuan Hong, Sk Jeesun

Published 2026-09-15

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CC BY 4.0

Original authors: Xiao-Gang He, Xuan Hong, Sk Jeesun

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

For decades, astronomers have known that the universe is filled with something invisible. This mysterious substance, called dark matter, does not emit light, but its gravity holds galaxies together and shapes the cosmic web. It makes up about a quarter of the universe's total energy, yet no one has ever directly seen a particle of it. Scientists have built massive, ultra-sensitive detectors deep underground to catch these elusive particles as they occasionally bump into atoms in the detector. For a long time, the leading theory was that dark matter consists of heavy, slow-moving particles that interact very weakly with normal matter. However, a recent discovery has shaken up this simple picture.

The Large Underground Xenon (LZ) experiment, a giant tank of liquid xenon buried deep in a mine, recently recorded a single, strange event. A particle struck the detector with a recoil energy of 248 kiloelectronvolts, a value far higher than any known background noise could produce. While this single flash of energy is tantalizing, it presents a puzzle. If the dark matter particle were the standard heavy type, it should have caused many more, smaller collisions at lower energies, which the detector did not see. To explain this lone high-energy hit without triggering a flood of low-energy ones, physicists had to look for a more exotic explanation involving a specific type of interaction where the dark matter particle changes its state upon impact.

A team of researchers, led by Xiao-Gang He, Xuan Hong, and Sk Jeesun, has proposed a new model to solve this riddle. They suggest that the dark matter particle is not a single, unchanging object, but rather exists in two forms: a lighter version and a slightly heavier version. When the heavier version collides with an atomic nucleus in the detector, it drops down to the lighter state, releasing extra energy that creates the high-energy signal seen by LZ. Crucially, this process is "exothermic," meaning it releases energy, which allows the heavier particle to survive from the early universe until today without being wiped out. This scenario requires the dark matter to interact with normal matter in a very specific way: it must talk to quarks, the building blocks of protons and neutrons, but ignore other particles like neutrinos or certain heavy quarks that would otherwise create conflicts with other astronomical observations.

To make this work, the researchers extended the standard model of particle physics by adding a new force carrier, a particle called a Z-prime boson. This new particle acts as a messenger between the dark sector and the visible world, but it only connects to the first two generations of quarks. This selective connection is vital because it prevents the dark matter from annihilating into neutrinos or other particles that would have been detected by telescopes looking for gamma rays from the sun or the center of the galaxy. The team calculated that if the universe reheated to a relatively low temperature after the Big Bang, this specific setup could naturally produce the right amount of dark matter. In this low-temperature scenario, the heavier and lighter dark matter particles are created in equal numbers, allowing the heavier one to persist and cause the single event observed in the detector.

The study carefully checked this idea against all known constraints. They found that there is a parameter space where the dark matter mass is around one trillion electronvolts and the mass difference between the two states is a few hundred kiloelectronvolts that can explain the observed relic density and the LZ event simultaneously. Furthermore, because the new force carrier does not interact with certain particles at the most basic level, the model naturally escapes the indirect detection limits. The researchers also addressed a potential problem: the new force could cause unwanted changes in the way particles mix and decay. They showed that by carefully arranging the mathematical structure of the model, these unwanted effects could be canceled out, leaving the theory consistent with all current particle physics data.

This work offers a coherent explanation for a single, baffling event that has stumped the scientific community. It suggests that dark matter might be more complex than a simple, static particle, possessing an internal structure that allows it to change states. While the model relies on specific conditions in the early universe and new particles that have not yet been directly observed, it provides a viable path forward. It demonstrates that a heavy dark matter particle, interacting through a specialized force with quarks, can explain the high-energy hit in the LZ detector without contradicting the silence of the rest of the universe. The findings invite further investigation, encouraging scientists to look for similar patterns in future data and to test the existence of the proposed new force carrier in high-energy colliders.

Technical Summary: Hadrophilic Inelastic Freeze-in Dark Matter in a $q_1-q_2$ Gauge Extension

Problem Statement
The recent observation of a single nuclear recoil event at $248 \pm 23 \pm 23$ keV by the LUX-ZEPLIN (LZ) collaboration, which exceeds known Standard Model (SM) backgrounds, has motivated the exploration of Beyond Standard Model (BSM) explanations. While Weakly Interacting Massive Particles (WIMPs) are a primary candidate, the standard WIMP paradigm faces tension: explaining a high-energy event typically requires a TeV-scale mass and interaction strength that would also produce a large flux of low-energy recoil events ( $\lesssim 100$ keV), contradicting null observations. Existing literature often proposes inelastic dark matter (DM) with endothermic scattering (where a lighter DM state $\chi_1$ up-scatters to a heavier state $\chi_2$ ), but these models frequently struggle with indirect detection constraints, particularly from solar capture and subsequent annihilation into visible final states (e.g., $b\bar{b}$ , $\nu\bar{\nu}$ ). Furthermore, ensuring the stability of the heavier DM component ( $\chi_2$ ) in thermal production scenarios is difficult, as $\chi_2$ typically decays rapidly to $\chi_1$ .

Methodology and Model Framework
The authors propose a specific BSM framework to address the LZ event while evading indirect detection constraints and ensuring the survival of the heavier DM component. The model features:

Gauge Extension: An abelian $U(1)_X$ gauge symmetry with a "hadrophilic" and flavor-dependent structure, specifically a $q_1-q_2$ symmetry where only the first two generations of quarks carry charge. The charges are assigned such that $u, d, c, s$ are charged, while third-generation quarks ( $t, b$ ) and leptons are neutral.
Mediator: A new gauge boson $Z'$ mediates interactions between the SM and the dark sector. Crucially, the absence of tree-level couplings between $Z'$ and neutrinos, $\tau$ leptons, or $b$ quarks naturally suppresses indirect detection signals.
Dark Sector: The dark sector consists of a singlet fermion $\chi$ and a singlet scalar $S$ . The scalar $S$ acquires a vacuum expectation value (VEV), generating a mass splitting $\delta m$ between two physical dark states, $\chi_1$ and $\chi_2$ . The interaction is exothermic, involving the down-scattering of the heavier state $\chi_2$ to $\chi_1$ ( $\chi_2 + N \to \chi_1 + N$ ).
Yukawa Sector: To generate the correct CKM matrix and quark masses without violating the $U(1)_X$ symmetry, the model introduces four additional Higgs doublets ( $H_{1-4}$ ) alongside the SM Higgs.
Production Mechanism: The authors explore a freeze-in production scenario at a low reheating temperature ( $T_{RH} < m_\chi$ ). Unlike thermal freeze-out, this non-thermal mechanism allows for the simultaneous production of $\chi_1$ and $\chi_2$ with comparable abundances, enabling the heavier component to survive to the present day.

Key Contributions and Analysis

Exothermic Scattering Dynamics: The paper demonstrates that exothermic scattering naturally suppresses low-energy recoil events. The kinematics of the process ( $\chi_2 \to \chi_1$ ) shift the recoil energy spectrum, allowing the model to explain the isolated high-energy event without overproducing events at lower energies.
Relic Density and Parameter Space: The authors solve the Boltzmann equations for the freeze-in scenario. They identify a region in the $Z'$ mass ( $M_{Z'}$ ) and coupling ( $g_{Z'}$ ) parameter space that simultaneously satisfies the observed relic density and the LZ event rate. They find that for a DM mass of $m_\chi \approx 1$ TeV, the required coupling is stronger than in standard freeze-in but weaker than in thermal freeze-out, compensating for the Boltzmann suppression at low $T_{RH}$ .
Flavor Constraints and FCNCs: The flavor-dependent nature of the $U(1)_X$ symmetry introduces potential Flavor Changing Neutral Currents (FCNCs), particularly in meson mixing (e.g., $B_s-\bar{B}_s$ ). The authors analyze these constraints and demonstrate that by appropriately choosing the rotation matrices in the quark sector (specifically the right-handed down-quark mixing matrix), the contributions to FCNCs can be canceled or suppressed to satisfy experimental limits.
Collider Constraints: The model is checked against LHC dijet search constraints (CMS and ATLAS). The allowed parameter space for the $Z'$ boson is shown to be consistent with current exclusion limits.

Results
The study identifies a viable parameter space where:

A TeV-scale DM particle ( $m_\chi \sim 1$ TeV) with a mass splitting $\delta m \sim \mathcal{O}(100)$ keV explains the LZ event.
The required spin-independent cross-section is approximately $O(2)$ smaller than that required in endothermic scattering models, reducing tension with low-energy constraints.
The model avoids indirect detection limits due to the lack of $Z'$ couplings to $b$ -quarks and neutrinos at tree level.
The freeze-in mechanism at low $T_{RH}$ ($60-140$ GeV) successfully populates both $\chi_1$ and $\chi_2$ components, resolving the stability issue of the heavier state.
The parameter space is consistent with constraints from meson oscillations (via specific matrix choices) and LHC dijet searches.

Significance
The paper claims to provide a consistent explanation for the isolated LZ event that resolves the tension between high-energy signals and low-energy null results, as well as indirect detection constraints. By combining a hadrophilic $q_1-q_2$ gauge extension with a low-temperature freeze-in production mechanism, the authors demonstrate a scenario where a heavy, inelastic DM component survives to the present day and produces the observed recoil. The work highlights the viability of flavor-dependent DM models and suggests that future studies should investigate flavor-dependent hadrophilic DM production with low reheating temperatures, moving beyond the assumption of instantaneous reheating. The authors also note that while a similar model appeared in Ref. [28], their work differs by considering freeze-in production and exothermic scattering in a distinct parameter space.

Technical Summary: Hadrophilic Inelastic Freeze-in Dark Matter in a q1−q2q_1-q_2q1​−q2​ Gauge Extension

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Technical Summary: Hadrophilic Inelastic Freeze-in Dark Matter in a $q_1-q_2$ Gauge Extension