⚛️ phenomenology

Is the Light Neutralino Dark Matter Still Viable in the MSSM After the LZ-2024 Results?

This paper demonstrates that the 2024 LZ results, when combined with updated nucleon matrix elements and complete one-loop corrections, definitively exclude sub-hundred-GeV Bino-like neutralino dark matter in the MSSM via $h$ - or $Z$ -resonant annihilation at statistical significances exceeding $4\sigma$ to $5.9\sigma$ .

Original authors: Zhiyang Bao, Subhadip Bisal, Junjie Cao

Published 2026-10-08

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CC BY 4.0

Original authors: Zhiyang Bao, Subhadip Bisal, Junjie Cao

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 more matter than we can see. Astronomers have long known that stars and galaxies move as if they are being pulled by an invisible hand, a substance that does not emit light and does not interact with the world around us in any ordinary way. This hidden substance is called dark matter, and it makes up roughly a quarter of everything in existence. For decades, physicists have searched for a specific type of invisible particle that could explain this cosmic weight. One of the most popular ideas was that dark matter consists of weakly interacting massive particles, or WIMPs. These hypothetical particles would be heavy enough to have survived from the birth of the universe, yet light enough to be produced in particle colliders, and they would occasionally bump into normal matter, leaving a faint trace that sensitive detectors could catch.

In the realm of theoretical physics, a framework called the Minimal Supersymmetric Standard Model offers a detailed map of how these particles might behave. This model suggests that for every known particle, there is a heavier, invisible partner. In this picture, the lightest of these new partners, a particle called the neutralino, is a prime candidate for dark matter. If this particle is light, weighing less than a hundred times the mass of a proton, it should have been easy to find. However, as experiments have grown more sensitive, the search has narrowed. The question now is whether the simplest version of this light particle, one that relies on specific, rare interactions to exist in the right amounts, can still survive the latest data.

A team of researchers from Zhengzhou University and Henan Normal University has taken a fresh, rigorous look at this possibility. They focused on a specific scenario where a light neutralino exists in a universe where its heavier cousins are so massive they are effectively out of reach. In this setting, the only way for the neutralino to have the correct amount of dark matter left over from the Big Bang is through a delicate balancing act. The particle must annihilate with its own kind at a very specific speed, a speed that is only possible if the particle's mass is tuned to be exactly half the mass of either a Higgs boson or a Z boson. These bosons are known force-carrying particles, and the neutralino would use them as a bridge to disappear. This mechanism is known as a resonance, similar to how a swing moves highest only when pushed at just the right rhythm.

The researchers realized that this same resonance that allows the dark matter to exist also dictates how often it should bump into normal atoms. If the particle is light enough to fit this specific mass range, it must interact with ordinary matter strongly enough to be detected, but not so strongly that it would have been seen already. The team set out to calculate exactly how strong this interaction should be, taking into account the most recent and precise measurements of how atomic nuclei behave. They included complex quantum corrections that had been overlooked in previous studies, which turned out to increase the predicted rate of these collisions by up to fifty percent. They also used the most conservative estimates for the internal structure of protons and neutrons, ensuring that their predictions were not overly optimistic.

With these refined calculations in hand, they compared their predictions against the latest results from the LUX-ZEPLIN experiment, a massive detector buried deep underground that is currently the most sensitive instrument for hunting dark matter. The results were decisive. The team found that the predicted interaction rates for these light neutralinos, even after accounting for every possible way to lower the signal, were still far higher than what the experiment observed. In fact, the data ruled out the possibility of this light particle existing in this specific way with a level of certainty that is staggering in the world of physics. The statistical significance of this exclusion exceeded five standard deviations for the Higgs-related scenario and more than four for the Z-related scenario. In scientific terms, this means the chance that these results are a fluke is less than one in a billion.

The study concludes that the simplest version of a light neutralino dark matter particle, one that relies solely on these resonant interactions to explain the universe's missing mass, is no longer viable. The window for this specific type of particle has been closed by the new data. This does not mean dark matter does not exist, nor does it rule out all forms of supersymmetry. It simply means that if this particle exists, it must be heavier, or it must interact with the world in a more complex way that avoids these specific resonant traps. The findings force physicists to look elsewhere, perhaps toward heavier particles or more intricate mechanisms, as the search for the true nature of the dark universe continues.

Technical Summary: Is the Light Neutralino Dark Matter Still Viable in the MSSM After the LZ-2024 Results?

Problem Statement
In the Minimal Supersymmetric Standard Model (MSSM) with heavy sfermions, a sub-hundred-GeV Bino-like neutralino ( $\tilde{\chi}^0_1$ ) faces a severe tension between achieving the observed dark matter (DM) relic abundance ( $\Omega h^2 \approx 0.12$ ) and satisfying direct detection constraints. Without light sfermions for co-annihilation, a pure Bino overcloses the universe. The only remaining thermal production mechanisms for a sub-100 GeV Bino-like $\tilde{\chi}^0_1$ are resonant annihilations via the $s$ -channel exchange of the Higgs boson ( $h$ -funnel, $m_{\tilde{\chi}^0_1} \approx m_h/2$ ) or the $Z$ boson ( $Z$ -funnel, $m_{\tilde{\chi}^0_1} \approx m_Z/2$ ). These resonances require a small Higgsino admixture to facilitate annihilation. However, the same couplings ( $C_h$ and $C_Z$ ) that govern the annihilation rate also determine the spin-independent (SI) and spin-dependent (SD) scattering cross-sections off nucleons. Consequently, reproducing the correct relic density imposes an irreducible lower envelope on the scattering cross-sections. The authors investigate whether the latest results from the LUX-ZEPLIN (LZ) experiment (LZ-2024) exclude these resonant scenarios, even when accounting for theoretical uncertainties and the possibility that the neutralino constitutes only a fraction of the total DM.

Methodology
The study employs a rigorous numerical scan of the MSSM parameter space, guided by an objective function that incorporates the relic density, direct detection limits, and various experimental constraints (Higgs properties, $B$ -physics, electroweak precision, and vacuum stability).

Theoretical Calculations:
- NLO Corrections: The authors compute complete one-loop supersymmetric corrections to the $\tilde{\chi}^0_1\tilde{\chi}^0_1h$ and $\tilde{\chi}^0_1\tilde{\chi}^0_1Z$ vertices. These corrections are propagated to the annihilation cross-sections and the nucleon scattering cross-sections ( $\sigma_{SI}$ and $\sigma_{SD}$ ). The study finds these corrections enhance the direct detection rates by up to 50% for SI and 30% for SD scattering compared to Leading Order (LO) predictions.
- Hadronic Inputs: The analysis adopts the FLAG-2024 lattice QCD averages for nucleon matrix elements ( $\sigma_{\pi N}$ , $\sigma_s$ , and spin fractions $\Delta q_N$ ). To ensure a conservative exclusion, the authors apply the most adverse $3\sigma$ downward shifts to these inputs, reducing the predicted cross-sections by 22% for SI and 38% for SD.
- Relic Density: The thermal relic abundance is calculated by solving the Boltzmann equation numerically using micrOMEGAs, incorporating the NLO vertices and thermal smearing effects near the resonances.
Statistical Analysis:
- Likelihood Recast: Instead of comparing against published exclusion curves, the authors perform a likelihood-based recast of the LZ-2024 data. They utilize micrOMEGAs (calibrated to a background-free Poisson model) and DDCalc (for LZ-2022) to evaluate the exclusion significance.
- Conservative Treatment: The analysis treats hadronic uncertainties as systematic downward rescalings of the central predictions rather than nuisance parameters to be marginalized. This ensures the resulting exclusion significance is a conservative lower bound.
- Scenarios: The study considers two cases: $\tilde{\chi}^0_1$ constituting 100% of DM ( $\Omega h^2 = 0.12$ ) and 50% of DM ( $\Omega h^2 = 0.06$ ).

Key Contributions

Complete NLO Vertex Corrections: The paper provides the first evaluation of the complete one-loop corrections to the $\tilde{\chi}^0_1\tilde{\chi}^0_1h/Z$ vertices in the context of the resonant funnel regions, quantifying their significant impact on direct detection rates.
Conservative Hadronic Uncertainty Handling: By adopting the FLAG-2024 matrix elements and applying the most adverse $3\sigma$ shifts, the authors establish a robust, conservative lower bound on the scattering cross-sections required to match the relic density.
Likelihood-Based Exclusion: The study moves beyond simple curve comparisons, employing a statistical likelihood framework to determine the Gaussian-equivalent significance of the exclusion, independent of LHC electroweakino search constraints.

Results
The authors find that the LZ-2024 results decisively exclude both resonant scenarios:

Exclusion Significance: Even after applying the most conservative theoretical and hadronic uncertainties (reducing cross-sections by up to 38%), the predicted scattering rates for the resonant regions lie well above the LZ-2024 limits.
- The $h$ -funnel is excluded at a conservative one-sided significance of $Z > 5.9\sigma$ ( $p \approx 1.8 \times 10^{-9}$ ).
- The $Z$ -funnel is excluded at a conservative one-sided significance of $Z > 4.1\sigma$ ( $p \approx 2.1 \times 10^{-5}$ ).
Independence: These exclusions hold whether the neutralino accounts for the total DM abundance or only a subdominant component. Furthermore, the exclusion is established independently of LHC electroweakino constraints (which typically favor $\mu \gtrsim 760$ GeV, whereas the $Z$ -funnel requires $\mu \lesssim 500$ GeV).
Comparison with Previous Data: While LZ-2022 results were insufficient for definitive exclusion (often yielding $Z < 2$ ), the increased exposure and sensitivity of LZ-2024 render the resonant regions untenable.

Significance
The paper concludes that the "light neutralino" thermal dark matter candidate in the MSSM, specifically the sub-100 GeV Bino-like neutralino relying on $h$ - or $Z$ -resonant annihilation, is effectively ruled out by current direct detection data. The authors argue that the fundamental tension arises because the LZ limits require the mediator coupling to be feeble, while the requirement to reproduce the relic abundance forbids the coupling from being arbitrarily small.

With the resonant funnels closed, the authors state that a thermal Bino-like neutralino in the MSSM must resort to alternative mechanisms, such as:

Co-annihilation (e.g., with staus or sleptons).
Heavy-Higgs resonance ( $A/H$ ).
A "well-tempered" Bino-Higgsino mixture (which typically pushes the mass scale above 200 GeV).

The study emphasizes that these conclusions apply to any Majorana DM candidate where the relic abundance and nucleon scattering originate from the same $h$ - or $Z$ -mediated interaction, provided no accidental cancellations or additional annihilation channels intervene.

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