⚛️ phenomenology

Can Nuclear Recoils and Solar Neutrinos Resolve the Composition of Singlino-Higgsino Dark Matter?

This paper proposes that combining nuclear recoil data from experiments like LUX-ZEPLIN with solar neutrino observations can resolve the composition of singlino-Higgsino dark matter in the NMSSM, as solar neutrinos provide critical constraints on ground-state configurations that nuclear recoils alone cannot distinguish.

Original authors: Jingwei Lian, Jin Min Yang

Published 2026-10-08

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Jingwei Lian, Jin Min Yang

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

The universe is filled with invisible matter that holds galaxies together, yet we have never seen a single particle of it. This "dark matter" makes up about a quarter of everything that exists, but it refuses to interact with light, making it impossible to photograph. For decades, scientists have built massive detectors deep underground, hoping to catch a dark matter particle bumping into an ordinary atom. When such a collision happens, it creates a tiny flash of light or a small jolt of energy, a signal known as a nuclear recoil. Recently, the LUX-ZEPLIN experiment, a giant tank of liquid xenon buried in a mine, reported seeing a candidate event at a specific energy level. This single, faint signal has sparked a global search to explain what kind of dark matter could cause it. The problem is that the same signal could be produced by very different types of particles, and the underground detector alone cannot tell them apart. It is like hearing a single knock on a door; you know someone is there, but you cannot tell if it is a friend, a stranger, or a delivery driver without looking through the peephole.

To solve this mystery, a team of researchers led by Jingwei Lian and Jin Min Yang turned their attention to a different kind of cosmic detector: the Sun. They focused on a specific theoretical framework called the Next-to-Minimal Supersymmetric Standard Model, which predicts that dark matter is made of a mixture of two types of particles: one that interacts only through a special force called the "singlino" and another that interacts through the weak nuclear force, known as the "Higgsino." In this model, these particles can exist in a ground state, which is their lowest energy level, and an excited state, which is a slightly higher energy level. The researchers investigated how these particles would behave if they were the ones causing the signal seen in the underground tank. They found that the type of dark matter determines not just how it hits the detector on Earth, but also how it gets trapped by the Sun's gravity and what happens to it once it is inside.

The team ran detailed computer simulations to track the journey of these particles. They calculated how dark matter from the galaxy would be captured by the Sun's immense gravity, sink to its core, and eventually annihilate with other dark matter particles. When these particles annihilate, they produce high-energy neutrinos, ghostly particles that can escape the Sun and travel to Earth. The researchers compared the predicted number of these solar neutrinos against ten years of data collected by the IceCube observatory in Antarctica, a massive detector that watches for neutrinos coming from the Sun. Their analysis revealed a clear distinction between the possible candidates. They found that if the dark matter is composed of a singlino ground state, it produces a signal in the underground tank that matches the LUX-ZEPLIN candidate, while simultaneously producing a low number of solar neutrinos that fits comfortably within the IceCube data. This scenario, where the singlino is the ground state, remains a viable explanation.

However, the study explicitly ruled out another possibility. If the dark matter were composed of a Higgsino ground state, the same signal in the underground tank would require the Sun to be a much brighter source of neutrinos than what IceCube has observed. The simulations showed that a Higgsino-based dark matter would create a solar neutrino signal so intense that it would contradict the actual data collected over a decade. This effectively eliminates the Higgsino ground state as the source of the event, provided the dark matter interacts in the specific way the researchers modeled. The study also explored a third scenario where the dark matter is a Higgsino pair, but in this case, the interaction would need to be purely elastic, meaning the particles bounce off without changing their internal energy state. This elastic version avoids the excessive neutrino problem by preventing the particles from being captured efficiently by the Sun in the first place, but it requires a very specific set of conditions to work.

The researchers concluded that by combining the data from the underground nuclear recoil detector with the data from solar neutrino observatories, they can distinguish between these different types of dark matter. The underground detector tells them how the particles hit the Earth, while the Sun acts as a filter that reveals how the particles behave when they are trapped and heated. This dual approach allows scientists to determine the composition of the dark matter halo, specifically whether the ground state is a singlino or a Higgsino. While the study relies on simulations and theoretical models, it offers a concrete path forward for future experiments. By looking at the same event through two different cosmic lenses, scientists can finally begin to identify the true nature of the invisible substance that fills our universe.

Technical Summary: Can Nuclear Recoils and Solar Neutrinos Resolve the Composition of Singlino-Higgsino Dark Matter?

Problem Statement
The LUX-ZEPLIN (LZ) collaboration has reported a nuclear-recoil candidate event at $248 \pm 23 \text{ (stat)} \pm 23 \text{ (sys)}$ keV. While nuclear recoil experiments measure the scattering of dark matter (DM) on nucleons, they often fail to uniquely determine the nature of the halo DM, particularly regarding its internal composition and mass ordering. In the Next-to-Minimal Supersymmetric Standard Model (NMSSM), different orderings of singlino ( $S$ ) and Higgsino ( $H$ ) neutralinos can produce nearly identical recoil spectra. Specifically, an interchange between the ground state and an excited state can preserve the inelastic recoil signature while drastically altering the ground-state annihilation cross-section. This creates a degeneracy where nuclear recoil data alone cannot distinguish between scenarios such as a singlino ground state with two Higgsino excitations (SHH) versus a Higgsino ground state with a singlino excitation (HSH). The paper investigates whether combining nuclear recoil data with solar neutrino constraints can resolve this composition degeneracy.

Methodology
The authors analyze the NMSSM in the CP-conserving, R-parity-conserving, $Z_3$ -invariant limit, focusing on the three lightest neutralinos composed of singlino and Higgsino states. They define three primary mass orderings:

SHH: Singlino ground state, followed by two Higgsinos.
HSH: Higgsino ground state, followed by a singlino excitation and a second Higgsino.
HHS: Two Higgsino ground states, with a heavier singlino-like state.

The analysis proceeds through the following steps:

Mixing and Splitting Relations: The authors derive analytic relations for the mixing matrix elements ( $N_{id}, N_{iu}, N_{iS}$ ) and the mass splittings ( $\delta_{21}$ ) for each ordering. They establish how the transition coupling ( $f = |C_{12}|^2$ ) and the Higgsino fraction ( $F_{H,i}$ ) relate to the physical parameters $\lambda$ , $\mu$ , and $\tan \beta$ .
Recoil Signal Modeling: They model the nuclear recoil spectrum using a truncated Maxwellian Standard Halo Model (SHM). The calculation includes coherent inelastic Z-mediated transitions, spin-independent (SI) elastic scattering, and spin-dependent (SD) scattering, utilizing Helm form factors for coherent interactions and the GCN5082 response for xenon SD interactions. The model is compared against the LZ 2.84 tonne-year exposure, including acceptance, resolution, and calibration profiling.
Solar Capture and Evolution: The authors simulate the capture of DM by the Sun, accounting for the opening of additional channels via solar gravity. They track the orbital evolution of captured particles, including excitation, de-excitation, decay, elastic cooling, escape, and annihilation. Crucially, they do not assume a thermal core or number equilibrium for inelastic solutions, allowing for a more general treatment of the captured population.
Solar Neutrino Constraints: The annihilation of captured DM produces high-energy neutrinos. The predicted neutrino flux is compared against the ten-year IceCube solar neutrino search results. Solar compatibility is defined by the ratio $R_{\odot} < 1$ , where $R_{\odot}$ represents the predicted signal relative to the IceCube limit.
Global Scanning: A parameter scan is performed over neutralino masses ( $200\text{--}2000$ GeV) and splittings ( $100\text{--}600$ keV), varying scalar and coannihilation parameters. The models are constrained by the Planck relic density, perturbativity, Higgs sector limits, flavor physics, and LHC searches.

Key Results

Resolution of SHH vs. HSH: The study identifies that while SHH and HSH orderings can produce similar high-energy inelastic recoil spectra, they are distinguished by their solar neutrino signatures, though HSH is not universally excluded.
- SHH (Singlino Ground State): In this scenario, the singlino ground state has a weak coupling to the Z boson ( $F_{H,1} \ll 1$ ). Consequently, the annihilation rate is suppressed ( $\langle \sigma v \rangle_{11} \propto f^2$ ), leading to a negligible solar neutrino signal. These models are compatible with both LZ and IceCube.
- HSH (Higgsino Ground State): Here, the ground state is Higgsino-dominated ( $F_{H,1} \approx 1$ ). While the transition to the excited state can be tuned to match the LZ recoil, the ground-state annihilation remains strong (electroweak strength). This typically leads to excessive solar capture and neutrino production. However, the analysis shows that HSH scenarios can survive solar constraints if the accessible transition to the third state is closed (e.g., due to a large mass gap to the same-sign Higgsino partner) and the high-energy recoil fit is slightly weaker. Specifically, benchmark point BP3c (HSH) survives with $R_{\odot} = 0.762\text{--}0.925$ , demonstrating that HSH is not automatically ruled out but is more tightly constrained than SHH.
The HHS Case: Inelastic HHS solutions (Higgsino ground states with a singlino excitation) generally fail the solar constraint due to strong coherent transitions and efficient annihilation. However, the authors identify an elastic HHS alternative (BP4). By closing the inelastic transition (large mass splitting $\sim 2.98$ GeV), the model suppresses solar capture. This scenario relies on elastic scattering (mediated by axial Z or scalar interactions) to explain the LZ event, which is consistent with solar limits but requires a different scattering mechanism than the inelastic cases.
Benchmark Points: The authors present four benchmark points (BP1–BP4):
- BP1 (SHH): A solar-compatible singlino ground state with a small mixing parameter ( $\lambda \sim 10^{-4}$ ) and suppressed annihilation.
- BP2c (SHH) vs. BP3c (HSH): These points illustrate the degeneracy in recoil spectra (masses and splittings differ by $<1.5\%$ ) but are distinguished by solar limits. BP2c (SHH) passes solar constraints due to weak annihilation, while BP3c (HSH) survives with $R_{\odot} = 0.762\text{--}0.925$ despite having strong Higgsino annihilation, provided the transition to the third state is kinematically closed.
- BP4 (HHS): An elastic scattering scenario that evades solar constraints by kinematically closing the inelastic channel.

Significance and Claims
The paper claims that nuclear recoil experiments alone cannot resolve the composition of singlino-Higgsino dark matter because different mass orderings can yield indistinguishable recoil spectra. However, the combination of recoil data with solar neutrino observations provides a critical discriminant:

Solar Neutrinos as a Probe: Solar neutrinos probe the ground-state annihilation properties and orbital evolution, which are decoupled from the transition coupling measured by nuclear recoils. This allows the exclusion of inelastic HHS scenarios and constrains HSH scenarios (which require specific kinematic conditions to survive), whereas SHH scenarios remain naturally compatible.
Mechanism Identification: The study highlights that distinguishing between inelastic and elastic scattering mechanisms is essential. While inelastic HHS is excluded by solar limits, an elastic HHS alternative remains viable, emphasizing the need for target complementarity (e.g., Xenon vs. Tungsten) to identify the scattering mechanism.
Model Constraints: The analysis demonstrates that resolving the composition requires joint constraints from direct detection, solar capture, and collider searches. The authors conclude that while residual degeneracies exist (set by halo assumptions and annihilation details), the specific combination of LZ and IceCube data significantly narrows the viable parameter space for NMSSM dark matter, specifically ruling out inelastic HHS and constraining HSH scenarios with strong ground-state annihilation unless specific kinematic conditions are met.

The paper does not claim to definitively identify the nature of the LZ event but rather establishes a framework where solar neutrino data is necessary to break the degeneracy inherent in nuclear recoil measurements for singlino-Higgsino systems.

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