⚛️ phenomenology

Nonthermal Solar Stalling of an Inelastic Scalar Signal in Xenon

This paper proposes an anomaly-free $U(1)_{B-3L_\tau}$ model featuring an inelastic scalar dark matter candidate that explains the LUX-ZEPLIN nuclear-recoil event while evading stringent solar-neutrino constraints due to suppressed solar capture and nonthermal stalling of the signal within the Sun.

Original authors: XinXin Qi, Hao Sun

Published 2026-09-11

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: XinXin Qi, Hao Sun

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 Earth's surface, in a tank of liquid xenon cooled to near absolute zero, scientists are listening for the faintest possible tap. This is the LUX-ZEPLIN experiment, a massive detector designed to catch dark matter, the invisible substance that makes up most of the universe's mass but refuses to interact with light. For decades, researchers have searched for these particles by waiting for them to bump into the nuclei of xenon atoms, creating a tiny flash of light and a small amount of heat. Recently, the experiment reported a single, intriguing event: a particle seemed to strike a xenon atom with an energy of 248 kiloelectronvolts. While this could be a random fluctuation, it sits at a high energy level that is difficult to explain with standard theories, prompting physicists to ask if this is the first glimpse of a new kind of dark matter.

To understand what might be happening, one must first grasp the nature of the search. Dark matter is thought to be a sea of particles drifting through our galaxy. When these particles pass through the Earth, they occasionally collide with atomic nuclei. In many theories, these collisions are simple bounces, like billiard balls hitting each other. However, some models suggest that dark matter particles have internal states, similar to how an atom can be in a ground state or an excited state. If a dark matter particle is in a lower energy state, it might need to absorb energy from a collision to jump to a higher state. This is called an endothermic transition. Such a process would require a specific, high-speed impact to occur, naturally filtering out lower-energy collisions and potentially explaining why the LUX-ZEPLIN detector saw a signal at a high energy while seeing very few at lower energies.

In a new study, researchers XinXin Qi and Hao Sun from Dalian University of Technology have constructed a detailed theoretical model to see if this specific scenario could explain the recent observation without breaking other laws of physics. They proposed a universe where dark matter consists of complex scalar particles, which are a type of fundamental particle, interacting through a new, very light force carrier. This force carrier acts like a messenger, transmitting forces between particles, but in this case, it is much lighter than the particles themselves. The researchers set the mass of their dark matter particle at 840 GeV, a value that is heavy enough to be a significant component of the universe's mass, and gave it an internal energy gap of 320 keV. This gap is the exact amount of energy needed to trigger the endothermic transition that would produce the 248 keV signal seen in the detector.

The team did not stop at matching the signal on Earth; they had to ensure their model survived the harsh environment of the Sun. Dark matter particles are constantly captured by the Sun's gravity, sinking toward its core. If these particles accumulate and collide with each other, they can annihilate, producing high-energy neutrinos that travel to Earth and can be detected by observatories like IceCube. For decades, the lack of such neutrinos from the Sun has been used to rule out many dark matter theories. The researchers needed to prove that their specific model would not produce a flood of neutrinos that would have already been seen. They calculated how these captured particles would behave inside the Sun, taking into account the Sun's heat, the motion of its atoms, and the fact that the dark matter particles could exist in two different internal states.

Their calculations revealed a surprising outcome. Because of the specific way their model's force carrier interacts with protons and neutrons, the capture of dark matter on iron atoms inside the Sun is almost completely suppressed. Iron is a major component of the Sun's core, and usually, it is the primary place where dark matter gets trapped. By effectively turning off the interaction with iron, the researchers prevented the dark matter from sinking deep into the Sun's center. Instead, the captured particles remained in a much wider, more diffuse cloud, orbiting at a distance roughly 20 percent of the way from the Sun's center to its surface. This distance is far too large for the particles to collide with each other frequently enough to produce the intense neutrino signals that IceCube would have detected.

The study shows that this specific arrangement of particles and forces can simultaneously explain the single event seen by LUX-ZEPLIN, account for the total amount of dark matter in the universe, and avoid the strict limits set by neutrino telescopes. The researchers also checked their model against data from the NA64 experiment at CERN, which searches for new particles by firing electrons at a target, and found their theory remained consistent with those results as well. By simulating the long-term behavior of these particles over the lifetime of the Sun, they demonstrated that the population of dark matter remains "stalled" in this extended orbit, never collapsing into the dense core where annihilation would be detectable.

This work provides a concrete example of how a specific type of dark matter could hide from our most sensitive detectors in the Sun while still leaving a trace in a terrestrial experiment. It suggests that the rules governing how dark matter interacts with the Sun are more complex than previously assumed, particularly when the particles have internal energy states and the force carrying the interaction is very light. While the single event seen by LUX-ZEPLIN is not yet confirmed as a discovery, this model offers a viable path forward, showing that the laws of physics can accommodate such a signal without contradicting the silence of the neutrino detectors. The researchers emphasize that their findings rely on detailed simulations of particle behavior and solar physics, establishing a benchmark that future experiments can test to see if this specific version of dark matter is indeed the one drifting through our galaxy.

Technical Summary: Nonthermal Solar Stalling of an Inelastic Scalar Signal in Xenon

Problem Statement
The LUX-ZEPLIN (LZ) experiment reported a single nuclear-recoil candidate event at $248 \pm 23$ (stat) $\pm 23$ (sys) keV within an extended energy search. While endothermic transitions are a natural mechanism to populate high-energy recoil regions while suppressing lower-energy backgrounds, standard interpretations often rely on thermal equilibrium assumptions or specific particle candidates like Higgsinos. A significant challenge in interpreting such signals is the "solar capture" constraint: dark matter captured by the Sun is expected to thermalize and annihilate in the core, producing high-energy neutrinos that are constrained by IceCube. Previous studies suggest that solar bounds on electroweak inelastic dark matter (such as Higgsinos) are stringent. However, it remains unclear whether these bounds transfer model-independently to light-mediator scalar realizations, particularly when nonthermal effects and specific nuclear interference patterns are considered.

Methodology
The authors construct a specific, anomaly-free $U(1)_{B-3L_\tau}$ model to test the viability of an inelastic scalar dark matter candidate against terrestrial, solar, and cosmological constraints.

Model Construction: The model introduces a complex scalar $S$ (splitting into real states $\chi_1$ and $\chi_2$ ) and a vector mediator $Z'$ associated with the $B-3L_\tau$ gauge symmetry. The charge assignment ( $Q_q = q$ for quarks, $-9q$ for $\tau$ -sector leptons) ensures the cancellation of all local gauge and mixed gravitational anomalies. A discrete $Z_2$ symmetry stabilizes the dark matter.
Benchmark Parameters: The benchmark features a dark matter mass $m_{\chi} \approx 840$ GeV with an inelastic splitting $\delta = 320$ keV. The vector mediator has a mass $m_{Z'} = 160$ MeV and a dark charge $Q_\chi = 1$ . The coupling $g_\chi \approx 1.056 \times 10^{-3}$ is tuned to match the LZ event rate.
Nuclear Physics: The model utilizes the physical proton-to-neutron coupling ratio $f_n/f_p = -0.86985$ . This specific ratio arises from the interference of the vector mediator's couplings to up and down quarks.
Solar Evolution Simulation: Unlike standard assumptions of isothermal cores, the authors perform a finite-rate, two-state orbit calculation. They evolve captured particles in energy, angular momentum, and internal state ( $\chi_1 \leftrightarrow \chi_2$ $χ_{1} \leftrightarrow χ_{2}$ ), incorporating:
- Finite collision rates and thermal nuclear velocities (Maxwellian distribution).
- The decay of the excited state $\chi_2 \to \chi_1 \nu_\tau \bar{\nu}_\tau$ .
- Solar profiles from multiple sources (legacy micrOMEGAs, SF-III/GS98, and SF-III/AGSS09).
Constraint Checking: The model is tested against:
- LZ: Reconstructed event rates in the 125–400 keV window.
- NA64: Recast limits on $B-L$ scenarios using bremsstrahlung and resonant production scaling.
- IceCube: Comparison of the predicted annihilation rate ( $\Gamma_{ann}$ ) against a public-data spectral proxy derived from IceCube's null results.
- Cosmology: Relic abundance ( $\Omega h^2$ ) calculation via freeze-out.

Key Results

Terrestrial Signal: The benchmark predicts $0.958$ accepted events in the 125–400 keV range for the LZ experiment, consistent with the observed single event at 248 keV. The sideband (350–590 keV) contains negligible background ($0.04$ events), supporting the signal's spectral shape.
Solar Capture Suppression: The specific $f_n/f_p$ ratio causes a near-perfect cancellation of the capture amplitude on Iron ( $^{56}\text{Fe}$ ), which typically dominates solar capture. Consequently, the total capture rate is $C_\odot = 1.09 \times 10^{19} \text{ s}^{-1}$ , suppressed by a factor of $\sim 355$ compared to a pure proton-current benchmark. Calcium and Nickel become the dominant capture targets.
Nonthermal Stalling: The finite-rate orbit evolution reveals that the captured population does not thermalize to the solar core. Instead, the distribution "stalls" at an equivalent radius $r_{eff} = 0.194\text{--}0.198 R_\odot$ . This is significantly larger than the critical radius required to satisfy IceCube constraints ( $r_{crit} \approx 0.024 R_\odot$ ).
Neutrino Constraints: Due to the extended distribution, the annihilation rate is $\Gamma_{ann} = (1.33\text{--}1.38) \times 10^{15} \text{ s}^{-1}$ . This is more than three orders of magnitude below the IceCube public-data proxy limit ( $\Gamma_{90}^{IC} \approx 1.8\text{--}1.97 \times 10^{18} \text{ s}^{-1}$ ). Even under conservative stress tests (varying solar composition, halo parameters, and metal density), the annihilation rate remains well below the exclusion limit.
Consistency: The model simultaneously satisfies the NA64 $B-L$ limits, reproduces the correct relic abundance ( $\Omega h^2 \approx 0.120$ ), and passes tree-level boundedness and global vacuum tests.

Significance and Claims
The paper claims to identify a concrete "loophole" in the transfer of solar bounds from Higgsino interpretations to general inelastic dark matter models. Specifically, it demonstrates that:

Solar-neutrino bounds on electroweak inelastic dark matter do not transfer model-independently to light-mediator scalar realizations.
A combination of isospin interference (suppressing iron capture) and nonthermal solar evolution (preventing core thermalization) allows a model to accommodate the LZ event, relic abundance, and accelerator limits while evading stringent solar neutrino constraints.
The result establishes the existence of an anomaly-free thermal benchmark that is viable across multiple experimental frontiers, challenging the assumption that a single inelastic operator or thermal core approximation suffices for all dark matter interpretations.

The authors remain modest regarding statistical significance, noting that the LZ event interpretation requires an official detector likelihood to define a confidence region and that the IceCube comparison relies on a public spectral proxy rather than an event-level likelihood. However, the three-order-of-magnitude separation from the IceCube limit is presented as robust against normalization uncertainties.

More like this