⚛️ phenomenology

Solar Capture Tests of Inelastic Dark Matter after the LZ High-Recoil Event

This paper demonstrates that while Solar capture constraints definitively exclude the thermal Higgsino interpretation of the recent LZ high-recoil event, they do not generically rule out other endothermic dark matter models, such as thermal pseudo-Dirac fermions or neutron-philic spin-dependent scattering, which remain viable despite being significantly below current IceCube limits.

Original authors: Mattia Di Mauro, Halim Shaikh

Published 2026-09-09

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CC0 1.0

Original authors: Mattia Di Mauro, Halim Shaikh

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

Most of the matter in the universe is invisible. It does not shine, it does not reflect light, and it does not interact with the atoms that make up our world in any way we can easily detect. Astronomers call this substance dark matter. While we know it exists because its gravity holds galaxies together, we have never seen a single particle of it. For decades, scientists have searched for these particles by building massive detectors deep underground, hoping to catch a dark matter particle bumping into an atom in a tank of liquid xenon. Recently, one such experiment, the LUX-ZEPLIN detector, reported a single, puzzling event. A heavy atom in the tank seemed to have been struck with far more energy than any standard theory of dark matter would predict. This single flash of energy has sparked a new wave of theories, suggesting that dark matter might not be a simple, static particle, but something more complex that changes its internal state when it hits an atom.

A team of researchers has now taken a closer look at this mysterious event, asking a critical question: if this strange signal is indeed caused by a new type of dark matter, could we have already seen the consequences of that same dark matter trapped inside our own Sun? The Sun acts as a giant gravitational trap. As dark matter particles from the galaxy drift past, the Sun's immense gravity pulls them in. If they collide with the dense material at the Sun's core, they can lose enough speed to become trapped, orbiting inside the star for billions of years. Over time, these trapped particles should collide with each other and annihilate, releasing a flood of high-energy neutrinos that travel straight to Earth. By calculating how many dark matter particles the Sun should have caught and how many neutrinos they should produce, the researchers tested whether the new theories explaining the underground event could survive this solar test.

The study focused on three specific ideas about what this dark matter might be. The first idea involves a particle known as a thermal Higgsino, a heavy, stable particle that physicists have long expected to exist. The second idea suggests the dark matter is a "pseudo-Dirac" fermion, a particle that exists in two slightly different energy states, switching between them when it interacts. The third idea proposes a particle that interacts specifically with the spin of neutrons, a property that makes it behave very differently in the dense environment of the Sun compared to the xenon tank on Earth. The researchers built detailed computer models to simulate how each of these three candidates would behave if captured by the Sun, accounting for the extreme heat and density of the solar core, which can actually help particles overcome energy barriers that would stop them in a cold underground detector.

The results were decisive for the first candidate. The thermal Higgsino, which fits the underground event perfectly, was found to be impossible if it also exists in the amounts required by the universe's history. The researchers calculated that if this particle were real, the Sun would have captured so many of them that they would have annihilated and produced a massive burst of neutrinos. However, the IceCube neutrino observatory in Antarctica, which watches the Sun for exactly this kind of signal, has seen nothing. The predicted signal is far too strong compared to what is actually observed. The study concludes that the thermal Higgsino explanation for the underground event is effectively ruled out; the Sun would have given it away long ago.

The other two ideas, however, tell a different story. For the pseudo-Dirac particle and the neutron-focused particle, the situation is much more subtle. In these models, the dark matter particles inside the Sun get stuck in a complex dance of energy states. They can be excited to a higher energy level by a collision, but they can also drop back down. The researchers found that for these specific types of particles, the process of them annihilating and creating neutrinos is heavily suppressed. Even though the Sun captures a significant number of these particles, the internal mechanics prevent them from building up the dense, hot core needed to produce a detectable neutrino signal. The predicted number of neutrinos remains far below the limits set by the IceCube observatory.

This means that while the Sun has acted as a powerful filter, eliminating the simplest and most predictive version of the new dark matter theory, it has not closed the door on the more complex possibilities. The study demonstrates that the strange event seen underground could still be caused by these more exotic forms of dark matter, provided they have the specific internal structure that keeps them from lighting up the Sun. The researchers did not find a new discovery, but they did perform a rigorous stress test, showing that the universe is consistent with some of these new ideas while firmly rejecting others. The search for dark matter continues, now guided by the knowledge that if the answer lies in a complex, shifting particle, the Sun is a place where that particle can hide in plain sight.

Technical Summary: Solar Capture Tests of Inelastic Dark Matter after the LZ High-Recoil Event

Problem Statement
The LUX-ZEPLIN (LZ) collaboration reported a candidate nuclear-recoil event with a reconstructed energy of $E_R = 248 \pm 23 \text{ (stat)} \pm 23 \text{ (sys)}$ keV. This event exhibits a local significance of $3.4\sigma$ (reduced to $2.6\sigma$ globally) and lies far above the energy window typical for standard elastic spin-independent WIMP searches. Several inelastic dark matter (DM) interpretations, specifically those involving endothermic scattering ( $\chi_1 A \to \chi_2 A$ with mass splitting $\delta > 0$ ), provide high local significances for this event.

While terrestrial direct detection (DD) experiments like LZ are sensitive to these inelastic signatures, the Sun offers a complementary probe. Gravitational acceleration within the Solar potential allows DM particles to reach velocities significantly higher than those in terrestrial detectors, potentially enabling the capture of DM with larger mass splittings ( $\delta$ ) that are kinematically inaccessible on Earth. Once captured, DM can accumulate in the Solar core and annihilate, producing high-energy neutrinos constrained by the IceCube Neutrino Observatory. The central problem addressed is whether the specific inelastic DM models favored by the LZ event are excluded by these Solar neutrino limits, particularly considering the complex post-capture evolution of multi-state DM systems.

Methodology
The authors perform a detailed analysis of Solar capture, orbital evolution, and annihilation for three distinct inelastic DM benchmarks:

Thermal Higgsino: A nearly pure Higgsino with a mass near 1.1 TeV, where the splitting $\delta$ is determined by electroweak symmetry breaking.
Thermal Pseudo-Dirac Fermion: A vector-interaction model with a mass of 1 TeV and a splitting of $\delta \approx 297$ keV, tuned to reproduce the LZ rate and thermal relic abundance.
Neutron-Philic Endothermic Spin-Dependent (SD) Scattering: An axial-vector interaction model ( $O_4$ operator) with $m_\chi = 1$ TeV and $\delta = 300$ keV, designed to test the complementarity between xenon (neutron-rich) and Solar (trace odd-neutron isotopes) targets.

The methodology involves three distinct physical stages:

Capture: The authors calculate the capture rate ( $C_\odot$ ) using the BS05(AGS,OP) Standard Solar Model. They generalize the standard Gould formalism to include inelastic kinematics, accounting for the energy cost $\delta$ and the binding condition. Crucially, for the neutron-philic case, they include finite-temperature effects, averaging over the Maxwell-Boltzmann velocity distribution of Solar nuclei to account for thermal re-excitation of heavier, trace odd-neutron isotopes (e.g., $^{57}\text{Fe}$ , $^{73}\text{Ge}$ ) that are kinematically closed in stationary-target approximations.
Post-Capture Evolution: Unlike standard elastic WIMPs, inelastic DM requires tracking the internal state populations ( $N_1$ and $N_2$ ) and orbital distribution. The authors distinguish between "stalled" orbits (where inelastic up-scattering becomes kinematically forbidden) and fully thermalized populations. They incorporate loop-induced elastic scattering (Spin-Independent and Spin-Dependent) which, despite small cross-sections, can drive secular cooling over the Solar age ( $t_\odot \approx 4.57$ Gyr).
Annihilation and Neutrino Signal: The annihilation rate $\Gamma_A$ is calculated without assuming instantaneous equilibrium ( $\Gamma_A = C_\odot/2$ ). For the Pseudo-Dirac case, they solve coupled rate equations for the two-state system, explicitly calculating the thermally assisted re-excitation rate $\Gamma_{12}$ and de-excitation rate $\Gamma_{21}$ . The resulting neutrino flux is mapped to IceCube limits, using the $W^+W^-$ channel as a proxy for the Higgsino and a $b\bar{b}$ channel proxy (with a $WW$ stress test) for the non-Higgsino benchmarks.

Key Contributions and Results

Thermal Higgsino Exclusion:
- The LZ-preferred splitting for a thermal Higgsino is $\delta \approx 377$ keV.
- The authors find that even in the most conservative "no-elastic-cooling" scenario (where inelastic up-scattering stalls at $r \approx 0.2 R_\odot$ ), the maximum allowed splitting to satisfy IceCube limits is $\delta_{\text{lim}} \approx 512.5$ keV.
- Including loop-induced elastic cooling (SI and SD) drives the population toward thermal equilibrium ( $R_\chi \approx 0.0037 R_\odot$ ), raising the exclusion limit to $\delta_{\text{lim}} \approx 565$ keV.
- Conclusion: The thermal Higgsino interpretation of the LZ event is robustly excluded by Solar capture constraints, as the required $\delta$ lies well below the IceCube exclusion bound in all cooling scenarios.
Pseudo-Dirac Vector Fermion Viability:
- For the benchmark ( $m_\chi = 1$ TeV, $\delta = 297$ keV), the Solar capture rate is $C_\odot \approx 1.48 \times 10^{20} \text{ s}^{-1}$ .
- While this rate is high, the annihilation is suppressed by the two-state kinetics. The excited state $\chi_2$ is required for annihilation ( $\chi_1 \chi_2 \to q\bar{q}$ ), but the population of $\chi_2$ is dynamically small.
- Thermally assisted re-excitation ( $\chi_1 \to \chi_2$ ) is frequent on extended orbits ( $\Gamma_{12} t_\odot \gg 1$ ), but the de-excitation rate $\Gamma_{21}$ is orders of magnitude faster ( $\Gamma_{21} \gg \Gamma_{12}$ ).
- Result: The maximum annihilation rate is $\Gamma_A \simeq 4.1 \times 10^{14} \text{ s}^{-1}$ , which is approximately $1.8 \times 10^5$ times below the IceCube $b\bar{b}$ limit. The model remains viable because the excited state does not accumulate sufficiently to produce a detectable neutrino signal.
Neutron-Philic SD Scattering Viability:
- For $m_\chi = 1$ TeV and $\delta = 300$ keV, finite-temperature capture including heavier trace isotopes yields $C_\odot \approx 3.98 \times 10^{17} \text{ s}^{-1}$ (with a range of $(1.77–9.72) \times 10^{17} \text{ s}^{-1}$ ).
- Even assuming equilibrium (a conservative upper bound for this model), the annihilation rate remains a factor of $\sim 154$ below the IceCube $b\bar{b}$ limit.
- Result: The model is not excluded. The separation between the strong neutron-spin response in xenon (driving the LZ signal) and the weak capture in the Sun (due to the scarcity of suitable odd-neutron targets and kinematic thresholds) protects the model.

Significance and Claims
The paper claims that Solar capture constraints are a powerful discriminator for inelastic DM models motivated by the LZ high-recoil event.

Exclusion: The study definitively excludes the canonical full-density thermal Higgsino interpretation of the LZ event. The required mass splitting is too small to evade the Solar neutrino bounds, regardless of uncertainties in loop-induced elastic cooling.
Survival: Conversely, the paper demonstrates that Solar constraints do not generically exclude endothermic explanations of the LZ candidate. Specifically, models where the annihilation requires a specific excited-state population (Pseudo-Dirac) or where the Solar capture is suppressed by target mismatch (Neutron-Philic SD) can survive the IceCube limits.
Methodological Insight: The authors emphasize that simple equilibrium assumptions ( $\Gamma_A = C_\odot/2$ ) are insufficient for inelastic DM. The viability of these models depends critically on the detailed kinetics of the two-state system (re-excitation vs. de-excitation rates) and the thermal motion of Solar nuclei, which can reopen kinematic channels closed in stationary-target approximations.

The paper concludes that while the thermal Higgsino is ruled out, the broader class of endothermic DM remains a viable explanation for the LZ event, provided the particle physics realization avoids the specific conditions that lead to rapid thermalization and efficient annihilation in the Solar core.

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