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Interpreting the High-Recoil LUX-ZEPLIN Event with Bino-/Singlino-like and Higgsino Dark Matter

This paper interprets a recent high-recoil nuclear event observed by the LUX-ZEPLIN experiment through two supersymmetric scenarios: an elastic scattering model involving a bino- or singlino-like neutralino with specific Higgsino asymmetry, and an inelastic scattering model featuring a pseudo-Dirac Higgsino, both of which predict distinct recoil spectra that can be distinguished from the candidate signal.

Original authors: Subhojit Roy, Pedro Schwaller, Carlos E. M. Wagner

Published 2026-10-01

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CC0 1.0

Original authors: Subhojit Roy, Pedro Schwaller, Carlos E. M. Wagner

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

Deep in the silence of the universe, a mysterious substance known as dark matter is thought to hold galaxies together, yet it remains invisible to our telescopes and undetectable by our most sensitive instruments. Scientists believe this substance is made of particles that rarely interact with ordinary matter, passing through the Earth like ghosts. To catch a glimpse of them, researchers have built massive detectors filled with liquid xenon, waiting for a dark matter particle to occasionally bump into a xenon atom. When such a collision happens, it should give the atom a tiny kick, or recoil, releasing a flash of light that the detector can record. The challenge is that these kicks are incredibly faint and can be easily confused with background noise from natural radioactivity. Recently, the LUX-ZEPLIN experiment, a state-of-the-art detector buried deep underground, reported a single, unusual event: a xenon atom that received a surprisingly strong kick, far more energetic than the typical whispers expected from dark matter. This single flash of light, occurring at an energy of about 248 kiloelectronvolts, has sparked a new wave of investigation into what kind of invisible particle could deliver such a powerful blow.

Two teams of physicists have now proposed two very different explanations for this event, both rooted in a theoretical framework called supersymmetry, which suggests that every known particle has a heavier, hidden partner. The first explanation imagines a dark matter particle that is mostly a "bino" or a "singlino," types of heavy partners that interact with the xenon atom through a specific magnetic-like force. In this scenario, the particle hits the atom and bounces off elastically, meaning it does not change its own internal state. The second explanation is more complex: it suggests the dark matter particle is a "Higgsino," a partner related to the Higgs boson, which arrives at the detector in a lighter state and then absorbs energy from the collision to transform into a heavier version of itself. This process, called inelastic scattering, is like a ball hitting a wall and using the impact to jump to a higher shelf; it requires a minimum amount of speed to happen, which changes the pattern of how often these collisions occur at different energies.

The researchers behind this study did not claim to have solved the mystery of the event. Instead, they built detailed computer models to see how each of these two theories would behave across the entire range of energies the detector can measure. They found that while both theories could potentially produce the single strong kick observed, they predict completely different patterns for the rest of the data. The first theory, involving the bino or singlino, predicts that if a strong kick happens at high energy, there should also be a significant number of weaker kicks happening at lower energies. It is as if a heavy stone thrown into a pond creates a large splash, but also sends out many smaller ripples that spread out across the water. The second theory, involving the Higgsino changing states, predicts the opposite: the energy required to change the particle's state acts as a filter, suppressing the weaker kicks entirely. In this case, the detector would see the single strong event but very few, if any, of the smaller ripples that the first theory expects.

To test these ideas, the scientists looked closely at the specific properties of the particles they proposed. For the first scenario, they calculated that the dark matter particle must have a very specific internal structure to interact with the xenon atom in the way observed, while simultaneously avoiding detection by other experiments that look for different types of interactions. They constructed a detailed model using the Next-to-Minimal Supersymmetric Standard Model, a complex extension of the standard laws of physics, which successfully creates a particle with the right properties. This model predicts a particle with a mass of about 500 gigaelectronvolts, a value that fits the observed event while remaining consistent with other known constraints. For the second scenario, the researchers explored how a Higgsino particle could gain the tiny mass difference needed to jump to a heavier state. They found that this difference could be generated by interactions with other heavy particles at extremely high energy scales, potentially as high as a few million gigaelectronvolts. However, they also discovered a significant problem with this second idea: if the Higgsino makes up all the dark matter in the universe, the same physics that allows it to jump states would also cause it to get trapped inside the Sun, where it would annihilate and produce high-energy neutrinos that our telescopes should have already seen. The fact that we have not seen these neutrinos makes this specific version of the Higgsino theory highly unlikely, unless the dark matter in our neighborhood is only a small fraction of the total dark matter in the universe.

The study concludes that the two theories leave distinct fingerprints on the data that future observations can distinguish. If the dark matter is the bino or singlino type, the detector should eventually record a steady stream of lower-energy events that correlate with the high-energy one. If it is the Higgsino type, those lower-energy events should be absent, and the signal should be concentrated only at the high-energy end, heavily dependent on the speed of the dark matter particles passing through the Earth. The researchers emphasize that their work is a guide for what to look for next, rather than a final verdict. The single event reported by LUX-ZEPLIN is not yet enough to declare a discovery, but it provides a unique opportunity to test the limits of our theories. By gathering more data and watching for the presence or absence of those lower-energy ripples, scientists will be able to tell which of these two very different stories, if either, is actually playing out in the dark matter that surrounds us.

Problem Statement
The LUX-ZEPLIN (LZ) experiment recently reported a nuclear-recoil candidate event at a reconstructed energy of $E_R \simeq 248$ keV. While the statistical significance is insufficient to claim a discovery (global significance of 2.6 $\sigma$ ), the unusually high recoil energy makes it a valuable probe for dark matter (DM) scenarios that predict enhanced support in the high-energy recoil tail. Standard elastic spin-independent (SI) interactions are suppressed at this energy due to nuclear form factors and the loss of coherence, which would typically predict a much larger excess at lower energies that is not observed. This paper investigates two supersymmetric interpretations based on distinct neutralino couplings to the $Z$ boson that can accommodate the high-energy event while remaining consistent with lower-energy data: (1) elastic scattering via a diagonal axial- $Z$ coupling (bino- or singlino-like DM), and (2) inelastic endothermic scattering via an off-diagonal vector- $Z$ coupling (pseudo-Dirac Higgsino DM).

Methodology
The authors perform a recoil-level analysis comparing the predicted spectral shapes of these two mechanisms across the extended LZ energy range, without performing a full multidimensional likelihood fit or global model reconstruction.

Framework: The analysis utilizes a Standard Halo Model (SHM) with a truncated Maxwellian distribution and evaluates recoil rates using the LZ extended-energy exposure ($2.84$ tonne-yr).
Elastic Scenario: The authors model a Majorana neutralino (bino- or singlino-like) with a diagonal axial- $Z$ coupling. They determine the coupling strength required to produce an order-one yield in the high-recoil region ($100-270$ keV) and quantify the correlated yield at lower energies ($5.4-100$ keV). They employ the GCN5082 xenon one-body density matrices as the reference nuclear response, with Anand–Fitzpatrick–Haxton (AFH) functions used for comparison.
Inelastic Scenario: The authors model a pseudo-Dirac Higgsino pair ( $\chi_1, \chi_2$ ) undergoing endothermic scattering ( $\chi_1 \to \chi_2$ ). They determine the mass splitting ( $\delta$ ) required to normalize the rate in the $215-269.9$ keV interval for reference masses of $500$ GeV and $1.1$ TeV. They analyze how the spectrum depends on the mass splitting, the dark matter mass, and the high-velocity tail of the halo distribution.
Supersymmetric Realization: The recoil properties are mapped to the underlying neutralino parameters. For the elastic case, the authors analyze the conditions in the MSSM and NMSSM required to generate the necessary axial coupling ( $X = |N_{13}|^2 - |N_{14}|^2$ ) while suppressing Higgs-mediated SI scattering (avoiding the "blind spot" where the axial coupling vanishes). For the inelastic case, they investigate the origin of the sub-MeV Majorana splitting via gaugino mixing (MSSM) or singlino mixing (NMSSM) and confront these scenarios with solar-capture neutrino constraints.

Key Contributions and Results

Spectral Discriminants:
- Elastic: The elastic axial- $Z$ interaction predicts a substantial correlated recoil yield at lower energies. For a $500$ GeV benchmark normalized to the high-energy candidate, the ratio of low-to-high recoil yields ( $R_{low}$ ) is approximately $5.3$ (using GCN5082). This "low-energy continuation" is a distinct signature.
- Inelastic: Endothermic scattering strongly suppresses the low-energy spectrum due to the kinematic threshold. However, the high-energy tail ( $>270$ keV) is highly sensitive to the dark matter mass and the extreme high-velocity tail of the halo. The $500$ GeV and $1.1$ TeV benchmarks predict markedly different high-recoil yields (differing by $\sim 90-94\%$ in the $350-590$ keV range), a distinction that is not halo-independent.
Elastic Supersymmetric Realization (Bino/Singlino-like):
- The recoil normalization fixes the Higgsino asymmetry $X$ rather than the total Higgsino fraction.
- In the MSSM, achieving the required $X \sim 0.01$ while suppressing SI scattering favors low $\tan\beta$ ( $\sim 2.1-2.2$ ). This creates tension with the observed Higgs mass ($125$ GeV) unless stop masses are very heavy (multi-TeV).
- In the $Z_3$ -NMSSM, the tree-level quartic Higgs coupling relaxes the Higgs mass constraint. The authors present a benchmark ( $m_\chi \simeq 505$ GeV, $X \simeq 0.007$ ) where the LSP is bino-dominated with a small Higgsino admixture.
- Relic Abundance: The benchmark achieves the correct thermal relic abundance via a singlet-like pseudoscalar resonance ( $a_1$ ) just above the neutralino pair threshold ( $m_{a_1} \approx 2m_\chi$ ). This mechanism suppresses present-day annihilation (solving indirect detection constraints) while allowing efficient freeze-out.
- Constraints: The benchmark satisfies current LHC electroweakino and heavy Higgs searches and lies below current LZ spin-dependent limits and solar-neutrino constraints (IceCube).
Inelastic Supersymmetric Realization (Pseudo-Dirac Higgsino):
- A sub-MeV splitting required for the $248$ keV event can be generated by electroweak gaugino mixing at the PeV scale (MSSM) or via singlino mixing (NMSSM).
- Solar Neutrino Constraints: The canonical thermal Higgsino interpretation ( $m_\chi \approx 1.1$ TeV) with a splitting of $\sim 376$ keV is strongly disfavored by solar-neutrino searches (IceCube/Super-K), which require splittings $\gtrsim 566$ keV to avoid excessive capture and annihilation.
- The $500$ GeV inelastic benchmark also faces tension with solar-capture constraints under standard assumptions, though non-thermal production histories or modified mixing could alter these bounds.

Significance and Claims
The paper claims that the two mechanisms offer "qualitatively different spectral continuations" away from the $248$ keV candidate, serving as the central spectral diagnostic for distinguishing them.

The elastic mechanism is characterized by a sizeable low-energy recoil population.
The inelastic mechanism is characterized by a suppressed low-energy population and a high-energy tail that is highly sensitive to the dark matter mass and the halo velocity distribution.

The authors emphasize that their analysis remains at the "recoil level" and does not reconstruct the full LZ likelihood. They state that a quantitative assessment of experimental discrimination requires the full detector response and background model. The paper concludes that while the elastic bino-like NMSSM benchmark provides a consistent ultraviolet realization that satisfies current constraints, the minimal thermal Higgsino interpretation is strongly constrained by solar-neutrino data. Future LZ exposure and improved sensitivity to spin-dependent scattering are identified as necessary to test these complementary spectral predictions.

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