⚛️ phenomenology

From LUX-ZEPLIN to Colliders: Probing Higgsino Dark Matter

This paper proposes that a specific high-energy nuclear recoil event observed by the LUX-ZEPLIN experiment can be explained by a 1.1 TeV Higgsino inelastic dark matter model with a 350 keV mass splitting, which predicts a nearly degenerate chargino that decays into a sub-centimeter tracklet within the detector, a signature currently unconstrained by the LHC but testable at future high-energy colliders.

Original authors: Kingman Cheung, Sin Kyu Kang, Ranjeet Kumar

Published 2026-09-09

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Kingman Cheung, Sin Kyu Kang, Ranjeet Kumar

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

For decades, astronomers have known that the universe is filled with invisible matter. This "dark matter" does not emit light, nor does it reflect it, yet its gravitational pull holds galaxies together and shapes the cosmic web. While we can see its effects, the fundamental nature of this substance remains one of the greatest mysteries in physics. One of the leading ideas is that dark matter consists of heavy, slow-moving particles that rarely interact with ordinary matter. If these particles exist, they should occasionally bump into the nuclei of atoms in deep underground detectors, creating tiny flashes of energy. The challenge for scientists is that these collisions are incredibly rare and often produce signals so faint they are easily lost in the background noise of the natural world.

Recently, a massive experiment called LUX-ZEPLIN, buried deep beneath the earth in South Dakota, reported a single, puzzling event. The detector recorded a nuclear recoil with an energy of approximately 248 keV. This is unusually high for a dark matter collision, which typically produces much weaker signals. While this single event is not enough to claim a discovery, it has sparked intense interest because it fits a specific theoretical model involving a type of particle called a Higgsino. If this model is correct, it implies that dark matter particles are not just heavy, but they also have a very specific internal structure that changes how they interact with the universe.

A team of physicists has now taken this intriguing hint and traced its consequences all the way to the world's most powerful particle colliders. They explored a scenario where the dark matter particle is a nearly pure Higgsino with a mass of about 1.1 TeV. In this model, the dark matter particle exists in two slightly different states that are almost identical in weight but separated by a tiny gap. When a dark matter particle from space hits an atom in a detector, it must gain enough energy to jump from the lighter state to the heavier one. This "inelastic" jump requires a high-speed collision, which explains why the LUX-ZEPLIN detector saw such a high-energy event. The researchers calculated that for this to happen, the mass difference between these two states must be roughly 350 keV.

The story does not end with the dark matter particle itself. The same theory that predicts these two neutral states also predicts a third, charged partner. This charged particle, known as a chargino, is almost the same mass as the neutral dark matter particles but is slightly heavier by about 350 MeV. Because this mass difference is so small, the charged particle cannot decay into heavy particles. Instead, it decays into a neutral dark matter particle and a very soft, low-energy pion, which is a type of subatomic particle. This process happens incredibly fast, with the charged particle living for only about 0.025 nanoseconds before vanishing.

This fleeting existence creates a unique signature for scientists to look for. When such a charged particle is created in a collider, it travels a tiny distance—roughly 6 to 8 millimeters—before it decays. In the language of particle physics, this is a "disappearing track." The particle leaves a short, visible trail in the detector's tracking layers, and then the trail simply stops. The decay product, the soft pion, is often too weak to be seen by standard detectors. This makes the signal extremely difficult to find, as the particle disappears before it can travel far enough to be fully reconstructed by the machine's sensors.

The researchers examined whether current experiments at the Large Hadron Collider (LHC) could have already found this particle. They found that the answer is no. The LHC has been searching for disappearing tracks, but its current sensitivity only extends to particles with masses up to about 225 GeV. The Higgsino predicted by the LUX-ZEPLIN event is much heavier, at 1.1 TeV, and the probability of it surviving long enough to be seen by the current detectors is vanishingly small. The team showed that the current LHC data does not rule out this scenario, leaving the door wide open for this specific type of dark matter.

Looking forward, the paper outlines how future machines could finally catch this elusive particle. The High-Luminosity LHC, which will operate with much more data in the coming years, might be able to see this signal, but only if the detectors are upgraded to recognize even shorter tracks. The researchers emphasize that simply collecting more data is not enough; the detectors must be able to reconstruct tracks that are just a few millimeters long. Even more promising are plans for future colliders, such as a 100 TeV proton collider or a 10 TeV muon collider. These machines would produce these heavy particles much more frequently and with higher speeds, increasing the chances that the charged particle travels far enough to be detected before it decays.

The beauty of this work lies in how it connects two very different ways of studying the universe. The LUX-ZEPLIN experiment looks for dark matter by watching for rare collisions in a tank of liquid xenon, while particle colliders try to create these particles from scratch. The paper demonstrates that if the LUX-ZEPLIN event is indeed caused by this specific Higgsino, then the charged partner must exist with a very precise mass and a very precise lifetime. This creates a clear target for future experiments. If a collider finds a disappearing track with a lifetime of about 0.025 nanoseconds and a mass of 1.1 TeV, it would provide independent confirmation of the dark matter signal seen underground. Conversely, if future colliders with the right capabilities fail to find this particle, the Higgsino explanation for the LUX-ZEPLIN event would be ruled out.

Ultimately, this research transforms a single, ambiguous data point into a concrete roadmap for discovery. It suggests that the universe might be hiding a heavy, charged particle that lives for a fraction of a billionth of a second, leaving behind only a microscopic scar in the detector. Whether this particle exists or not, the path to finding it is now clearly defined. The next generation of experiments will need to be built with the ability to see these tiny, disappearing trails, turning a theoretical possibility into a testable reality. If the universe is indeed filled with these heavy Higgsinos, the answer will not come from a single flash of light, but from the absence of a track that should have been there.

Technical Summary: From LUX-ZEPLIN to Colliders: Probing Higgsino Dark Matter

Problem Statement
The LUX-ZEPLIN (LZ) collaboration recently reported a single high-energy nuclear recoil event with a recoil energy of $E_R \approx 248$ keV. While statistically insufficient for a discovery (global significance $2.6\sigma$ ), the unusually high recoil energy suggests a potential interpretation via endothermic inelastic dark matter (DM). Specifically, this event hints at a model where a DM particle transitions to a slightly heavier state, requiring a mass splitting ( $\delta$ ) of approximately 350 keV. Theoretical analyses suggest a nearly pure Higgsino with a mass near 1.1 TeV as a viable candidate for this scenario. However, a critical phenomenological gap exists: while direct detection probes the neutral state splitting, the implications for the charged Higgsino (chargino) sector and its detectability at colliders remain to be fully characterized. Current Large Hadron Collider (LHC) searches for disappearing tracks have not yet excluded the parameter space required for a thermal 1.1 TeV Higgsino, leaving the viability of this interpretation untested by collider experiments.

Methodology
The authors analyze the Minimal Supersymmetric Standard Model (MSSM) in the limit of nearly pure Higgsinos, where the bino and wino masses ( $|M_1|, |M_2|$ ) are significantly larger than the Higgsino mass parameter ( $|\mu|$ ).

Mass Splitting Calculation: The study numerically diagonalizes the neutralino and chargino mass matrices to determine the mass spectrum. They impose the constraint that the neutral state splitting ( $\delta m^0$ ) must be $\approx 350$ keV to match the LZ event kinematics. This requires specific gaugino mass configurations, including a cancellation region where $M_1$ and $M_2$ have opposite signs or large magnitudes ( $O(10^2)$ TeV).
Chargino Properties: The authors calculate the charged-neutral mass splitting ( $\Delta m^+$ ), which is dominated by electroweak radiative corrections rather than tree-level mixing. They derive the resulting chargino lifetime and decay modes, focusing on the dominant decay $\tilde{\chi}^\pm_1 \to \tilde{\chi}^0_1 \pi^\pm$ .
Collider Phenomenology: The paper evaluates the production cross-sections at hadron colliders (LHC, HL-LHC, and 100 TeV pp) and muon colliders. It models the detector signatures, specifically the "short track" or "tracklet" formed by a chargino traveling a macroscopic distance before decaying. The analysis incorporates geometric acceptance, initial-state radiation (ISR) requirements for triggering, and the probability of the chargino surviving through specific pixel layers to form a reconstructible track.

Key Contributions and Results

Spectrum Prediction: For a thermal Higgsino mass of $m_{\tilde{\chi}^0_1} \approx 1.1$ TeV and a neutral splitting of $\delta m^0 \approx 350$ keV, the model predicts a charged-neutral mass splitting of $\Delta m^+ \approx 340\text{--}350$ MeV. This results in a chargino proper lifetime of $\tau \approx 0.02\text{--}0.03$ ns, corresponding to a decay length of $c\tau \approx 6\text{--}8$ mm.
Current LHC Constraints: The authors demonstrate that the current LHC sensitivity (ATLAS disappearing track and CMS soft isolated track searches) is insufficient to probe this benchmark. The combination of a TeV-scale mass (suppressing production cross-sections to $\sim 0.57$ fb at 13 TeV) and the short decay length leads to an exponentially small probability of the chargino surviving through enough detector layers to form a reconstructible track (e.g., a 3-hit track requires survival to $\sim 9$ cm, with a survival probability of $\sim 10^{-6}$ for typical boosts). Consequently, the 1.1 TeV benchmark remains unconstrained by current data, which only excludes Higgsinos up to $\sim 225$ GeV.
Future Prospects:
- HL-LHC: Increased luminosity alone is insufficient due to the geometric suppression. The authors argue that sensitivity depends critically on optimizing short-track reconstruction (e.g., reducing the minimum radial extent required for a tracklet) and correlating short tracks with soft pions.
- 100 TeV Proton Collider: A future 100 TeV collider with $\sim 30$ ab $^{-1}$ luminosity could probe the thermal Higgsino region, provided the inner tracker is optimized for short tracks.
- Muon Collider: A 10 TeV muon collider with $\sim 10$ ab $^{-1}$ luminosity is projected to achieve a $5\sigma$ discovery of a 1.1 TeV Higgsino, as the fixed center-of-mass energy and cleaner kinematics facilitate the detection of such short-lived states.
Model Trajectory: The paper establishes that in the decoupled-gaugino limit, the chargino mass and lifetime are not independent but lie on a specific one-parameter trajectory in the $(m_{\tilde{\chi}^\pm_1}, c\tau)$ plane. The LZ-motivated benchmark sits on or near this trajectory, making it a highly constrained target for collider searches.

Significance
The paper claims that the LZ high-energy recoil event provides a "motivated benchmark" rather than definitive evidence, but it serves as a concrete target for testing the Higgsino DM hypothesis. The primary significance lies in connecting direct detection observations (neutral splitting) with collider signatures (charged splitting and decay length). The authors emphasize that the specific combination of a TeV-scale mass and a sub-centimeter decay length creates a "challenging" but testable scenario. They conclude that while the scenario evades current bounds, it is within the reach of future high-energy colliders, provided that detector designs and analysis strategies are explicitly optimized for sub-centimeter tracklets. This work underscores the complementarity of direct detection and collider experiments in probing the specific mass splitting hierarchy inherent to the Higgsino DM framework.

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