⚛️ phenomenology

The LZ event with a two-state dark matter halo

This paper reanalyzes the LUX-ZEPLIN (LZ) experiment's single nuclear-recoil candidate by incorporating a two-state dark matter halo model, demonstrating that the presence of a small fraction of excited dark matter states can significantly lower the required scattering cross-section and alter kinematic constraints compared to standard interpretations.

Original authors: Vinicius Oliveira

Published 2026-09-29

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Vinicius Oliveira

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

Dark matter is the invisible substance that holds galaxies together, making up about a quarter of the universe's total energy. While its gravitational pull is undeniable, scientists have never directly detected a particle of dark matter interacting with ordinary matter. To find it, researchers build massive, ultra-sensitive detectors deep underground, waiting for a dark matter particle to occasionally bump into an atomic nucleus and create a tiny flash of light. The LUX-ZEPLIN experiment, located in a former gold mine in South Dakota, recently reported seeing a single, promising event that looked exactly like such a collision. This single flash, occurring at a specific energy level, has sparked intense debate because standard theories of dark matter struggle to explain why only one event appeared at that high energy while the expected low-energy signals were missing.

A researcher led by Vinícius Oliveira has revisited this single event to see if the standard explanation holds up when a specific, often-overlooked possibility is considered. The prevailing idea to explain the event involves "inelastic" dark matter, where a particle must absorb energy to change into a heavier version of itself before it can collide with an atom. This process, known as endothermic scattering, naturally suppresses low-energy collisions and pushes the signal to higher energies, matching the single event seen by LUX-ZEPLIN. However, this theory assumes that the dark matter halo surrounding our galaxy consists entirely of these lighter, ground-state particles. The new study challenges this assumption by asking what happens if the halo also contains a significant number of the heavier, excited-state particles.

In many theoretical models, both the light and heavy versions of the dark matter particle are created in the early universe and should both survive to the present day. If the heavier version exists in our galactic neighborhood, it can do the opposite of the standard scenario: instead of needing to absorb energy to collide, it can release energy as it drops down to its lighter state. This "exothermic" process would allow even the slowest, coldest dark matter particles to create a collision, potentially generating a much stronger signal than the endothermic version. The researcher rebuilt the statistical analysis of the LUX-ZEPLIN data from scratch, using public information to verify their methods against the official results, and then applied this new framework to a dark matter halo containing a mix of both particle types.

The findings reveal that the composition of the dark matter halo is critical to interpreting the event. If even a tiny fraction of the halo consists of the heavier particles, the dynamics change completely. The study shows that if the heavier particles make up more than a very small percentage of the total dark matter—ranging from one in a hundred thousand to one in a hundred, depending on the mass difference between the two states—the energy-releasing collisions would dominate the signal. In a scenario where the halo is split evenly between the two types, the amount of dark matter needed to explain the single observed event drops dramatically, by factors ranging from eighty to over ten thousand, compared to the standard model. Furthermore, the sharp cutoff in energy that limits the standard theory disappears, allowing the signal to extend to much higher energies.

This work suggests that previous interpretations of the LUX-ZEPLIN event, which assumed the dark matter halo contained only the lighter particles, may be incomplete. A specific theoretical model that perfectly explained the event under the old assumptions is now ruled out if the halo contains an equal mix of both particle types; in that case, the model predicts a signal far too strong to be compatible with the data. The researcher concludes that when scientists try to understand inelastic dark matter signals, they cannot ignore the possibility that the heavier, excited state of the particle survives in our galaxy. The single event observed is not just a puzzle to be solved by one type of particle, but a potential clue that the dark matter around us is a complex mixture, where the presence of a subdominant population of heavier particles could fundamentally alter our understanding of what we are seeing.

Technical Summary: The LZ Event with a Two-State Dark Matter Halo

Problem Statement
The LUX-ZEPLIN (LZ) experiment reported a single nuclear-recoil candidate at an energy of $E_R \simeq 248$ keV. While the collaboration interpreted this event through the lens of inelastic dark matter (iDM) undergoing endothermic scattering ( $\chi_1 N \to \chi_2 N$ ), this interpretation assumes the dark matter halo consists exclusively of the ground state $\chi_1$ . In pseudo-Dirac models, where the excited state $\chi_2$ is long-lived, the present-day halo naturally contains a mixture of both states. In such scenarios, exothermic scattering ( $\chi_2 N \to \chi_1 N$ ) becomes kinematically allowed and contributes to the signal. The published LZ statistical intervals and the simplified analyses found in recent literature do not account for this two-state halo composition, potentially misinterpreting the required cross-sections and kinematic features of the signal.

Methodology
To address this gap, the author reconstructs the LZ statistical inference using public data and applies it to a halo containing a fraction $f$ of the excited state $\chi_2$ .

Statistical Reconstruction: Since the full two-dimensional unbinned likelihood densities are not public, the author constructs a binned likelihood based on the public projection of LZ data (Fig. 5 of Ref. [4]). The data is projected onto three slices in the prompt scintillation signal ( $S1_c$ ) and binned by the distance to the nuclear recoil (NR) band in $\log_{10} S2_c$ . The likelihood function incorporates observed counts, post-fit background estimates, and the signal shape derived from the LZ detector response model (NEST).
Validation: The reconstructed analysis is validated against the published LZ intervals for the endothermic case ( $\chi_1$ only). The author confirms agreement within 26% for $\delta \geq 50$ keV, with minor deviations attributed to the loss of energy resolution inherent in binning and differences in the treatment of nuisance parameters.
Nuclear Response: The author utilizes a shell-model nuclear form factor (from dmdd) rather than the standard Helm form factor, correcting it to match the LZ spectral response at $\delta=0$ to ensure consistency with the experiment's specific nuclear physics inputs.
Kinematic Modeling: The scattering rate is calculated for a mixture of endothermic ( $\chi_1 \to \chi_2$ ) and exothermic ( $\chi_2 \to \chi_1$ ) processes. The exothermic process has no kinematic cutoff and peaks at a recoil energy fixed by the mass splitting $\delta$ , whereas the endothermic process is suppressed at low energies and possesses a sharp kinematic cutoff.

Key Contributions

Reconstruction of LZ Inference: The paper provides a validated, binned likelihood framework that reproduces LZ's published intervals for endothermic iDM, serving as a baseline for testing alternative halo compositions.
Two-State Halo Analysis: It is the first application of the full LZ statistical machinery to a halo containing a fraction $f$ of the excited state $\chi_2$ , moving beyond simplified normalizations or separate treatments of the two scattering channels.
Identification of Critical Fraction: The analysis identifies a critical fraction $f_{crit}$ (ranging from $10^{-5}$ to $10^{-2}$ depending on $\delta$ ) above which the exothermic contribution dominates the signal, fundamentally altering the interpretation of the event.

Results

Dominance of Exothermic Scattering: For a halo with equal abundances of $\chi_1$ $χ_{1}$ and $\chi_2$ $χ_{2}$ ( $f=1/2$ $f = 1/2$ ), the exothermic channel dominates the signal. This leads to a dramatic reduction in the required dark matter-nucleon cross-section ( $\sigma_N$ $σ_{N}$ ) to explain the single event.
- At $\delta = 250$ keV, the required cross-section drops by a factor of 80.
- At $\delta = 350$ keV, the drop is a factor of $1.6 \times 10^4$ .
Kinematic Changes: The presence of the exothermic component eliminates the kinematic cutoff characteristic of the endothermic signal. The allowed parameter space extends to higher mass splittings ( $\delta$ ) without the sharp drop-off seen in the $\chi_1$ -only scenario.
Exclusion of Standard Benchmarks: A thermal pseudo-Dirac benchmark ( $m_\chi \approx 1$ TeV, $\delta \approx 297$ keV, $\sigma_N \approx 6.5 \times 10^{-43}$ cm $^2$ ) that fits the LZ event under the assumption of a pure $\chi_1$ halo is excluded by a factor of 30 if the halo contains 50% $\chi_2$ .
Scaling Behavior: Above the critical fraction $f_{crit}$ , the required cross-section scales approximately as $1/f$ , indicating that even a small sub-dominant population of excited states significantly impacts the interpretation of direct detection limits.

Significance and Claims
The paper claims that the standard LZ interpretation, which assumes a halo composed solely of the ground state $\chi_1$ , is not directly applicable to scenarios where the excited state $\chi_2$ survives to the present day. The author demonstrates that the presence of even a sub-dominant excited-state population cannot be neglected. Specifically, if the halo contains a significant fraction of $\chi_2$ , the exothermic channel reduces the cross-section required to explain the LZ candidate by several orders of magnitude and removes the kinematic constraints associated with endothermic scattering. Consequently, previous limits and interpretations derived under the single-state assumption may not hold for pseudo-Dirac dark matter models with long-lived excited states. The author emphasizes that their results are a direct consequence of applying the correct statistical inference to a two-state halo, rather than a new experimental proposal or a claim of discovery.

Technical Summary: The LZ Event with a Two-State Dark Matter Halo

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