⚛️ phenomenology

The LZ inelastic splitting as a probe of anomaly mediation

This paper proposes a Minimal R-symmetric Supersymmetric Standard Model (MRSSM) framework where pseudo-Dirac dark matter, characterized by an inelastic Z interaction and anomaly-mediated mass splitting, offers a viable explanation for the LZ 248 keV recoil candidate while satisfying Higgs, gauge, and solar neutrino constraints.

Original authors: Seung J. Lee, Taewook Youn

Published 2026-10-12

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CC BY 4.0

Original authors: Seung J. Lee, Taewook Youn

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 hum of the universe, invisible particles known as dark matter are thought to drift through everything, from the space between stars to the atoms in our bodies. Scientists have spent decades trying to catch a glimpse of these ghosts, usually by waiting for them to bump into the nuclei of atoms in deep underground detectors. Most theories suggest these collisions happen at very low energies, like a gentle tap. However, a recent observation by the LUX-ZEPLIN experiment, a massive tank of liquid xenon buried in a mine, reported a single, unusual event. This event was a much harder hit, a recoil with an energy of 248 kiloelectronvolts, far higher than the typical background noise. This single flash of energy has sparked a new hunt for an explanation, leading physicists to consider a specific type of dark matter that does not just bounce off atoms but changes its internal state in the process, absorbing some of the energy to jump to a heavier version of itself.

A team of researchers has now proposed a detailed framework to explain this strange signal using a refined version of a theory called supersymmetry. In the standard version of this theory, the particles that make up dark matter are difficult to arrange in a way that fits the new data without creating contradictions with other known laws of physics. The researchers turned to a more specialized model, the Minimal R-Symmetric Supersymmetric Standard Model, which introduces new types of particles and symmetries. In this model, the dark matter particle is a hybrid, formed by pairing two different types of particles to create a stable, nearly identical twin. This pairing protects the particle's identity, but a tiny, subtle break in the symmetry allows the two twins to have slightly different masses, separated by just a few hundred thousand electronvolts. This tiny gap is exactly what is needed to explain the high-energy hit seen in the detector: the dark matter particle absorbs the extra energy to switch from its lighter state to its heavier twin.

The researchers showed that this specific arrangement of particles naturally avoids the problems that plagued earlier attempts. In previous models, getting the right mass difference required an unlikely coincidence of numbers, or it led to predictions that contradicted observations of how the sun captures dark matter. By using the new model, the team found that the dark matter particle can be heavy, around 730 gigaelectronvolts, and still produce the correct signal. They calculated that this particle would interact with the sun in a way that matches current limits, and it would also interact with other detectors in a way that could be confirmed or ruled out by future experiments. The model also predicts the existence of other, slightly heavier particles that would have been produced in the early universe, helping to explain why there is so much dark matter today.

To make this work, the researchers had to connect the tiny energy scale of the dark matter splitting to a much larger, hidden scale of energy in the universe. They used a concept involving extra dimensions of space, where the strength of forces changes as you move through a warped region. In this setup, the tiny mass difference is generated by a specific mechanism involving the curvature of space and quantum loops, which naturally suppresses the effect to the precise value needed. This approach keeps the dark matter particle stable and prevents it from decaying too quickly, while also ensuring that the other particles in the theory do not cause conflicts with known physics. The result is a self-consistent picture where the single event seen in the xenon detector is not a fluke, but a signature of a complex, hidden structure of particles.

The paper does not claim to have solved the mystery of dark matter, but it provides a concrete, testable path forward. The proposed model makes specific predictions about how often these particles should scatter off atomic nuclei and how they should behave inside the sun. If future experiments see more events at the same energy level, or if they detect the predicted heavier partners of the dark matter particle, this explanation would gain strong support. Conversely, if those signals are not found, this specific version of the theory can be discarded. The researchers emphasize that their solution is not a lucky guess but a result of carefully balancing the mathematical requirements of the theory with the physical constraints of the universe. They have mapped out a range of possible masses and interaction strengths, showing that a broad family of solutions exists that fits the current data. This work transforms a single, puzzling event into a roadmap for the next generation of experiments, offering a clear way to distinguish between different ideas about the invisible substance that shapes our cosmos.

Technical Summary: The LZ Inelastic Splitting as a Probe of Anomaly Mediation

Problem Statement
The paper addresses the interpretation of a single nuclear-recoil-like event at 248 keV reported by the LUX-ZEPLIN (LZ) experiment, with an expected background of 0.01 events. While various interpretations exist, this work focuses on inelastic dark matter (DM), specifically a pseudo-Dirac spectrum where the heavier neutral state is separated from the lighter one by a small mass splitting ( $\delta \sim 300$ keV). The scenario requires:

Suppressed Diagonal Coupling: The transition must be predominantly an off-diagonal vector interaction via the $Z$ boson, with the diagonal axial current suppressed to satisfy spin-dependent scattering bounds.
Correct Relic Abundance: The model must reproduce the observed thermal relic density ( $\Omega h^2 \approx 0.12$ ).
Naturalness: The small mass splitting must be technically natural, not relying on accidental cancellations of independent mass parameters.

The authors demonstrate that the Minimal Supersymmetric Standard Model (MSSM) fails to naturally realize this target. The MSSM Higgsino limit conflicts with solar-neutrino capture bounds, while the gaugino limit requires an unprotected mass degeneracy (fine-tuning at the level of $10^{-6}$ ) to suppress the transition strength.

Methodology and Framework
The authors propose a realization within the Minimal R-symmetric Supersymmetric Standard Model (MRSSM) embedded in a five-dimensional warped geometry (Randall-Sundrum type).

MRSSM Visible Sector:
- The model includes adjoint chiral multiplets for each gauge group and additional $R$ -Higgs doublets.
- This allows for Dirac gaugino masses (pairing the bino with the hypercharge adjoint fermion $\tilde{S}$ and the wino with the $SU(2)$ adjoint $\tilde{T}$ ), preserving a continuous $U(1)_R$ symmetry.
- The $U(1)_R$ symmetry forbids Majorana gaugino masses at tree level, protecting the near-degeneracy of the pseudo-Dirac pair.
- Coupling Relations: The authors impose the gauge-Higgs coupling relations of $N=2$ supersymmetry at a high scale ( $M_{IR} = 10^{14}$ GeV) and evolve them to low energy using two-loop MRSSM renormalization-group equations. This specific relation ( $\lambda_d = g'/\sqrt{2}$ ) ensures the mixing between the bino and down-type Higgsino suppresses the diagonal axial $Z$ current while allowing the required off-diagonal vector transition.
Warped Stabilization and Anomaly Mediation:
- To generate the small Majorana splitting without breaking the $R$ -symmetry at the TeV scale, the authors utilize anomaly mediation.
- The model is embedded in a 5D warped bulk where supersymmetry is broken by a bulk constant superpotential.
- Stabilization: The radion is stabilized via a Goldberger-Wise hypermultiplet.
- Local Compensator: The breaking of the no-scale Kähler structure (via a loop correction with coefficient $\beta$ ) generates a suppressed local compensator scale ( $F_{IR}$ ) on the infrared (IR) brane.
- Splitting Mechanism: The anomaly-mediated Majorana masses are proportional to $F_{IR}$ . The warped geometry suppresses this scale relative to the gravitino mass ( $m_{3/2}$ ), allowing $F_{IR} \sim 31$ MeV (required for a 300 keV splitting) even if $m_{3/2}$ is in the TeV range. This avoids the instability of the bino that would occur in a flat-space sequestered scenario.
Numerical Scan:
- The authors scan the dark matter mass ( $m_\chi$ ) and splitting ( $\delta$ ).
- Parameters such as the bino Dirac mass ( $M^D_B$ ), wino Dirac mass ( $M^D_W$ ), and down-type Higgsino mass ( $\mu_d$ ) are adjusted at each point to satisfy the LZ event rate, the relic abundance (via wino coannihilation), and the mixing strength ( $\sin^2\theta \approx 0.02$ ).
- Fixed inputs include $\tan\beta \approx 31$ , heavy scalar masses (20 TeV for adjoints, 10 TeV for squarks), and the high-scale matching conditions.

Key Results

Viable Spectra: The model produces viable spectra with dark matter masses spanning 300–850 GeV and splittings of a few hundred keV.
Benchmark Point: A specific benchmark is identified at $m_\chi = 730$ GeV and $\delta = 300$ keV.
- Mixing: The effective mixing strength is $\sin^2\theta \approx 0.02$ , consistent with the LZ event rate.
- Axial Suppression: The diagonal axial current is suppressed to $a_Z \approx -3 \times 10^{-5}$ , well below direct detection limits.
- Relic Density: The observed abundance is achieved through coannihilation with a nearby wino-like state (mass difference $\sim 11$ GeV).
- Elastic Scattering: The spin-independent elastic cross-section is $\sigma_n \approx 3.5 \times 10^{-48}$ cm $^2$ , which is below current LZ limits but within the projected reach of future experiments (XLZD).
- Solar Neutrinos: The predicted solar neutrino signal is approximately 0.59 times the IceCube limit, passing the adopted constraints.
Naturalness: The small splitting is technically natural, arising from the symmetry-breaking scale $F_{IR}$ rather than accidental cancellations.
Anomaly Mediation Scale: The paper establishes that a 300 keV splitting corresponds to a local anomaly-mediated scale of roughly 31 MeV. This probes the combination $m_{3/2} M_{IR} / M_{Pl}$ rather than the gravitino mass alone.

Significance and Claims
The paper claims to provide a technically natural realization of the inelastic dark matter interpretation of the LZ 248 keV candidate within the MRSSM. Its primary contributions are:

Resolution of MSSM Limitations: It overcomes the MSSM's inability to simultaneously satisfy the solar-neutrino bound and the required transition strength without fine-tuning.
Connection to Supersymmetry Breaking: It links the inelastic splitting directly to the mechanism of supersymmetry breaking (anomaly mediation) in a warped extra-dimensional setting. The small splitting is a direct probe of the local compensator scale generated by loop corrections in the bulk.
Testability: The model remains testable through:
- Elastic scattering (probing the Higgsino components).
- Solar neutrino observations (testing capture and annihilation).
- Long-lived charged states (searching for the coannihilating wino, though lifetimes are short at the benchmark point, they can be longer in other parts of the parameter space).
- High-energy recoil spectra in xenon and other target nuclei.

The authors emphasize that the model avoids the "accidental mass cancellation" of the MSSM gaugino realization and that the small splitting is protected by the approximate $U(1)_R$ symmetry, broken only by the calculable anomaly-mediated effects in the warped bulk.

Technical Summary: The LZ Inelastic Splitting as a Probe of Anomaly Mediation

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