⚛️ phenomenology

GUT-induced FCC signatures of the LUX-ZEPLIN event

This paper proposes that a recent LUX-ZEPLIN event can be explained by GUT-induced pseudo-Dirac dark matter, which predicts a $Z'$ boson with a mass in the tens of TeV range that is within the discovery reach of the Future Circular Collider (FCC-$hh$).

Original authors: Wojciech Kotlarski, Kamila Kowalska, Enrico Maria Sessolo

Published 2026-09-09

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CC BY 4.0

Original authors: Wojciech Kotlarski, Kamila Kowalska, Enrico Maria Sessolo

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 quiet dark of the universe, invisible particles known as dark matter are thought to drift through everything, including our bodies and the Earth. Scientists have long tried to catch these elusive particles by building massive detectors deep underground, hoping to see a tiny flash of light when a dark matter particle bumps into an atomic nucleus. Recently, a sophisticated experiment called LUX-ZEPLIN, buried in a mine in South Dakota, reported seeing a single, promising event. This was not a routine background noise; it was a collision with a surprisingly high amount of energy, suggesting that the dark matter particle involved might not be a simple, solid sphere, but rather a complex object that changes its internal state when it strikes. This specific type of interaction, where the particle gets a "kick" that splits it into two slightly different versions, has sparked intense debate among physicists about what kind of dark matter could cause such a signal.

The excitement around this single event lies in what it implies about the hidden architecture of the universe. If the dark matter particle is indeed a complex, split entity, standard theories of physics struggle to explain how it could exist without requiring other, even heavier particles that would be impossible to create in any machine we can currently build. However, a new study by researchers from the National Centre for Nuclear Research in Poland suggests a different path. They propose that if the universe follows a specific, grander design known as a Grand Unified Theory, the heavy particles required to create this split dark matter could be much lighter than previously thought. In fact, their calculations suggest these particles are light enough to be discovered by the next generation of particle colliders, specifically a massive machine called the Future Circular Collider, which is currently being planned.

The researchers focused on a specific scenario where the dark matter particle behaves like a "pseudo-Dirac" particle, a technical term for a particle that is almost, but not quite, its own mirror image. In the standard models of physics that many scientists favor, creating such a particle requires the existence of other, extremely heavy partners that are so massive they would be forever out of reach for any foreseeable experiment. The Polish team, however, looked at a different framework based on a mathematical structure called SU(6). In this framework, the rules of the universe are slightly different. They found that the same mechanism that creates the split in the dark matter particle also gives rise to a new type of force carrier, a heavy version of the photon called a Z-prime boson. Crucially, in this specific model, the mass of this new boson is directly tied to the size of the split in the dark matter.

By working through the mathematics of this specific model, the team discovered that the energy of the event seen by LUX-ZEPLIN points to a very specific mass for this new Z-prime boson: somewhere between 14.5 and 16 trillion electron volts. This is a massive amount of energy, far beyond what the current Large Hadron Collider can produce, but it sits comfortably within the design capabilities of the proposed Future Circular Collider, which aims to smash protons together at energies up to 100 trillion electron volts. The researchers calculated that if this model is correct, the new boson would decay into pairs of charged particles, such as electrons or muons, in a way that the Future Circular Collider would be able to spot with high confidence. They ran detailed simulations of what this signal would look like and found that it would stand out clearly against the background noise of the machine, offering a realistic chance for discovery in the early years of the collider's operation.

This work does not claim to have solved the mystery of dark matter, nor does it prove that the event seen by LUX-ZEPLIN was definitely caused by this specific type of particle. Instead, it offers a concrete, testable possibility. It suggests that if the dark matter particle is indeed the complex kind that caused the recent signal, then the universe likely contains a new force carrier that is just within our reach to find. The study effectively bridges the gap between a single, faint signal in a dark mine and the potential for a major discovery in a future machine, turning a speculative idea into a roadmap for experimental verification. If the Future Circular Collider is built and finds this new particle, it would not only confirm the nature of the dark matter seen in the mine but also validate a grander, unified view of how the fundamental forces of nature are connected.

Technical Summary: GUT-induced FCC Signatures of the LUX-ZEPLIN Event

Problem Statement
The LUX-ZEPLIN (LZ) collaboration recently reported an event consistent with a nuclear recoil energy of $248 \pm 23(\text{stat}) \pm 23(\text{sys})$ keV. The profile likelihood analysis favors a dark matter (DM) hypothesis over background, with a local significance of $3.4\sigma$ . The spectral shape strongly suggests inelastic scattering of a pseudo-Dirac DM candidate rather than standard elastic spin-independent scattering.

While a popular interpretation involves the inelastic scattering of a Higgsino from the Minimal Supersymmetric Standard Model (MSSM), this scenario faces a significant theoretical hurdle. Generating the required mass splitting ( $\delta m \approx 250\text{--}360$ keV) between neutral Higgsino states in the MSSM necessitates heavy gaugino masses ( $M_{1,2} \approx 10^7$ GeV). Such masses are far beyond the reach of any planned future collider, rendering the model effectively untestable via direct production of the heavy states responsible for the splitting.

Methodology
The authors propose an alternative framework rooted in Grand Unified Theories (GUTs) that accommodates a pseudo-Dirac DM candidate with Higgsino-like properties but avoids the "hopelessly out of reach" mass scales of the MSSM.

Model Construction: The study utilizes a specific non-supersymmetric GUT based on the $SU(6)$ gauge group. The fermion content is determined by anomaly cancellation requirements, involving copies of antifundamental ( $\bar{6}$ ) and antisymmetric ($15$) representations.
Symmetry Breaking and DM Stability: The model employs a breaking chain $SU(6) \to SU(5) \times U(1)_X \to \text{SM}$ . To ensure the stability of the DM candidate without ad-hoc discrete symmetries (like $Z_2$ ), the authors impose quantum scale symmetry (asymptotic safety) as a UV boundary condition. This symmetry forbids specific Yukawa couplings that would otherwise induce rapid decays of the DM candidate into Standard Model neutrinos.
Mass Splitting Mechanism: In this $SU(6)$ construction, the mass splitting $\delta m$ between the pseudo-Dirac DM states is generated by the vacuum expectation values (vevs) of scalar fields and specific Yukawa couplings. Crucially, the splitting is inversely proportional to the scale of the $U(1)_X$ breaking, which is tied to the mass of a new $Z'$ gauge boson.
Collider Analysis: The authors calculate the mass of the $Z'$ boson ( $m_{Z'}$ ) correlated with the experimentally favored $\delta m$ . They then perform a phenomenological study of the $pp \to Z' \to \ell^+\ell^-$ (dilepton) channel at the Future Circular Collider (FCC-hh) with $\sqrt{s} = 100$ TeV and an integrated luminosity of $30 \text{ ab}^{-1}$ . The cross-sections are computed at leading order using MadGraph5 aMC@NLO.

Key Contributions and Results

Correlation of Scales: The paper derives a parametric relationship showing that the experimentally favored mass splitting ( $\delta m \approx 340\text{--}360$ keV) for a $\sim 1.1$ TeV DM candidate implies a $Z'$ boson mass in the range of 14.5 to 16 TeV. This is derived from the relation $\delta m \propto v_{SM}^2 / v_{BSM}$ , where $v_{BSM}$ sets the $Z'$ mass.
FCC Reach: Unlike the MSSM gauginos, the $Z'$ boson in this $SU(6)$ GUT scenario falls squarely within the discovery potential of the FCC-hh. The authors show that for the predicted gauge coupling $g_X \approx 0.07$ (fixed by GUT boundary conditions), the signal cross-section for $pp \to Z' \to \ell^+\ell^-$ lies well within the projected 95% and 68% confidence level exclusion limits of the FCC.
Narrow Resonance: Due to the small predicted coupling ( $g_X \ll 1$ ), the $Z'$ resonance is very narrow, enhancing its detectability in the dilepton channel.
Additional Signatures: The model predicts the existence of vector-like charged particles (a tau lepton and a bottom quark) at the TeV scale. While their mixing with SM fermions is model-dependent, the authors note that even small mixing could allow for direct discovery in multilepton or lepton+jet channels at a 100 TeV collider.

Significance and Claims
The paper argues that if the LZ event is indeed caused by inelastic scattering of a Higgsino-like particle, the standard MSSM interpretation renders the underlying physics inaccessible to colliders. However, by embedding this scenario in a GUT framework with quantum scale symmetry, the mass splitting mechanism is linked to a $Z'$ boson with a mass of $\mathcal{O}(10)$ TeV.

The authors conclude that this specific GUT-induced scenario transforms the LZ observation from a potentially untestable anomaly into a "roadmap" for future discovery. They claim that the $Z'$ resonance associated with the LZ event "falls squarely in reach of the early discovery potential of the FCC-hh," offering a concrete, testable prediction for the next generation of hadron colliders. The work highlights that GUT constructions can naturally accommodate the required particle content and stability mechanisms while keeping the new physics scale accessible, unlike the MSSM gaugino scenario.

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