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Inelastic Singlet-Doublet Fermion Dark Matter in light of the 248 keV LZ event

Motivated by the recent LUX-ZEPLIN observation of a single 248 keV nuclear recoil event, this paper proposes an inelastic singlet-doublet fermion dark matter model extended with a scalar triplet, which naturally explains the absence of lower-energy events by suppressing elastic scattering while enabling inelastic transitions and simultaneously generating Majorana neutrino masses via the Type-II seesaw mechanism.

Original authors: Debasish Borah, Sujit Kumar Sahoo, Narendra Sahu, Shashwat Sharma

Published 2026-09-09

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CC BY 4.0

Original authors: Debasish Borah, Sujit Kumar Sahoo, Narendra Sahu, Shashwat Sharma

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 in the silence of the universe, invisible particles known as dark matter are thought to drift through everything, including our own bodies and the planet we stand on. While we cannot see them, scientists have long suspected that these particles occasionally bump into the atomic nuclei inside ordinary matter, creating a tiny, fleeting flash of energy. For decades, massive detectors buried deep underground have waited for these rare collisions, hoping to catch a glimpse of the invisible. Recently, the LUX-ZEPLIN experiment, a sophisticated detector filled with liquid xenon, reported something unusual: a single, high-energy collision that did not fit the expected pattern of background noise. This event, occurring at a specific energy level, sparked a new line of inquiry into what kind of particle could cause such a specific reaction, leading researchers to propose a model where dark matter behaves in a way that is slightly different from the standard theories.

The researchers behind this study focused on a specific type of dark matter candidate called a singlet-doublet fermion. In this model, the dark matter particle is not a single, unchanging entity but exists in two very similar states, like two versions of the same person that are almost identical in weight but not quite. The lighter version is the stable dark matter particle that fills the universe, while the heavier version is slightly more massive. Crucially, the rules of physics in this model prevent the dark matter from bouncing off a nucleus and staying in its original, lighter state. Instead, if a collision occurs, the particle must absorb a small amount of energy to jump up to the heavier state. This process, known as inelastic scattering, acts like a filter: it prevents low-energy collisions from happening while allowing only those with enough energy to trigger the jump. This mechanism naturally explains why the detector saw a single event at a high energy of 248 keV, while seeing nothing at lower energies where background noise usually hides.

To make this model work, the team introduced a new ingredient to the universe's particle zoo: a scalar triplet, a type of field that helps create the tiny difference in mass between the two dark matter states. This same field also plays a role in a completely different area of physics: the origin of neutrino masses. By linking the dark matter's mass difference to the mechanism that gives neutrinos their weight, the researchers created a unified picture where the mystery of dark matter and the mystery of neutrino mass are solved by the same underlying structure. They calculated how often these particles would collide with the xenon nuclei in the detector and found that the model could indeed reproduce the single event observed by the LUX-ZEPLIN collaboration. The calculations showed that for the right combination of particle mass and energy difference, the probability of this specific jump occurring matches the data.

However, the story is not without its constraints. While the model successfully explains the high-energy event, it must also survive the strict limits set by other experiments that have looked for dark matter bouncing off nuclei without changing its state. Because the same particle properties that allow the jump also allow for a tiny chance of a standard bounce, the researchers found that constraints on the elastic scattering rate strongly restrict the model's parameter space. They determined that only a narrow range of particle masses and mixing angles—how much the two dark matter states blend together—could satisfy both the new high-energy event and the existing rules that say no other collisions have been seen. Specifically, the mixing angle had to be small enough to keep the standard bounce rate below the detection threshold, which in turn forced the dark matter particle to have a mass in a specific range and the energy jump to be within a precise window.

The study concludes that this inelastic dark matter scenario remains a viable explanation for the recent LUX-ZEPLIN observation, provided the universe's parameters fall within these tight boundaries. The model offers a compelling narrative where a single particle type, split into two nearly identical states by a new field, can account for a mysterious signal while simultaneously explaining the origin of neutrino mass. As future detectors become more sensitive, they will be able to test these predictions further, either confirming this specific dance of particles or ruling it out. For now, the single event at 248 keV stands as a tantalizing clue, suggesting that the dark matter in our galaxy might be more complex and interactive than previously imagined, waiting to reveal its true nature through the rare, energetic jumps it makes when it finally strikes a nucleus.

Technical Summary: Inelastic Singlet-Doublet Fermion Dark Matter in light of the 248 keV LZ event

Problem Statement
The LUX-ZEPLIN (LZ) collaboration recently reported a single high-energy nuclear recoil event with an energy of $248 \pm 23_{\text{stat}} \pm 23_{\text{sys}}$ keV, observed over an exposure of 2.84 tonne-year. This event exhibits a tension with the background-only hypothesis at a global significance of $2.6\sigma$ . Standard Weakly Interacting Massive Particle (WIMP) models typically predict elastic scattering, which would populate lower recoil energies; the absence of such events at lower energies suggests the underlying process may be inelastic dark matter (DM)-nucleus scattering. The authors investigate whether the Singlet-Doublet Dark Matter (SDDM) model can explain this specific event while remaining consistent with constraints on DM relic abundance and elastic scattering limits.

Methodology and Model Framework
The authors extend the Standard Model (SM) with a vector-like fermion doublet $\Psi = (\psi^0, \psi^-)^T$ and a vector-like fermion singlet $\chi$ , both stabilized by a $Z_2$ symmetry.

Mass Generation and Splitting: After electroweak symmetry breaking, Yukawa interactions generate a Dirac mass mixing between $\chi$ and $\psi^0$ . To facilitate inelastic scattering, the model introduces a small Majorana mass term ( $m_1 \sim \mathcal{O}(100)$ keV) for the neutral component of the doublet. This splits the lightest DM candidate ( $\psi_1$ ) into two nearly degenerate pseudo-Dirac states, $\chi_1$ and $\chi_2$ , separated by a mass splitting $\delta = 2m_1$ .
Scalar Triplet Extension: The Majorana mass is generated dynamically by introducing a scalar triplet $\Delta \sim (1, 3, 1)$ , which is even under the $Z_2$ symmetry. This triplet couples to the fermion doublet and SM lepton doublets. Its vacuum expectation value (VEV) generates the required Majorana mass for the DM sector and simultaneously generates light neutrino masses via the Type-II seesaw mechanism.
Scattering Dynamics:
- Inelastic Channel: The Majorana nature of $\chi_1$ forbids tree-level $Z$ -mediated elastic scattering ( $\chi_1 N \to \chi_1 N$ ). However, it allows for inelastic up-scattering ( $\chi_1 N \to \chi_2 N$ ) via $Z$ -boson exchange, suppressed by the mixing angle $\sin^2\theta$ .
- Elastic Channel: Elastic scattering proceeds only via Higgs exchange, which is suppressed relative to the $Z$ -mediated inelastic process for the parameter space of interest.
Analysis: The authors calculate the differential recoil rate using the Standard Halo Model and the Helm form factor. They employ an extended maximum likelihood method to fit the model parameters ( $m_{DM}$ , $\delta$ , and $\sin\theta$ ) to the single observed LZ event.

Key Contributions and Results

Explanation of the 248 keV Event: The study demonstrates that the SDDM model can naturally explain the LZ event as an inelastic scattering process. The kinematic requirement for endothermic scattering ( $\chi_1 \to \chi_2$ ) naturally suppresses events at lower recoil energies, aligning with the observed spectrum.
Parameter Space Constraints:
- The analysis identifies preferred regions in the $(m_{DM}, \delta)$ plane for various mixing angles ( $\sin\theta$ ).
- As $\sin\theta$ decreases, the cross-section drops ( $\propto \sin^4\theta$ ). To maintain the observed event rate, the fit favors smaller mass splittings ( $\delta$ ) and lower DM masses ( $m_{DM}$ ). For instance, reducing $\sin\theta$ from 0.7 to 0.0135 shifts the preferred $\delta$ from $\sim 310\text{--}370$ keV down to $\sim 30\text{--}180$ keV.
- Benchmark Points: The authors provide specific benchmark points (e.g., $m_{DM} \sim 700$ GeV, $\sin\theta \sim 0.0135$ ) that satisfy the LZ event rate.
Consistency with Elastic Scattering Bounds: A critical finding is that while a large portion of the SDDM parameter space fits the LZ event alone, the stringent limits on elastic DM-nucleon scattering (from LZ and other direct detection experiments) strongly restrict the viable parameter space. Specifically, the authors show that benchmark points with $\sin\theta \lesssim 0.03$ (e.g., BP3 and BP4) are safe from current elastic scattering bounds while still accommodating the inelastic signal.
Neutrino Mass Connection: The model establishes a direct correlation between the small mass splitting in the dark sector (required for the LZ event) and the origin of neutrino masses, as both arise from the VEV of the same scalar triplet.

Significance and Claims
The paper claims to offer a viable particle physics interpretation for the recent LZ high-energy recoil event within the well-motivated Singlet-Doublet fermion framework. The significance lies in:

Natural Suppression of Elastic Scattering: The model inherently suppresses elastic scattering (forbidden at tree level for $Z$ -mediation and suppressed for Higgs mediation), allowing it to evade current null results for elastic scattering while still producing a detectable inelastic signal.
Unified Origin: It links the dark sector phenomenology (mass splitting) with the neutrino sector (Type-II seesaw) through a single scalar triplet extension.
Predictive Power: The authors conclude that while the model can explain the event, future constraints on elastic scattering rates will further narrow the allowed parameter space for the DM mass, pseudo-Dirac splitting, and mixing angle. The presence of the scalar triplet also opens additional avenues for detection and phenomenology beyond standard SDDM scenarios.

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