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The LZ event with a two-state dark matter halo

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Vinicius Oliveira

High Energy Physics - Phenomenology · Submitted 28 Sep 2026 · 60 references

Pith's verdict: The exothermic-dominance claim is kinematically robust and the recast is unusually transparent; the headline cross-section numbers, however, all sit on the unverified condition that chi2 lives 13.8 Gyr.

T0 · AI review

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arXiv ID2609.35650 (version 1)
Pith IDGV4TP6VK, Pith's permanent record of this paper and its review
CitationVinicius Oliveira. "The LZ event with a two-state dark matter halo." arXiv:2609.35650 (2026). https://pith.science/paper/GV4TP6VK
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The paper

The paper claims

Exothermic scattering from a two-state halo dominates LZ's 248 keV event once the excited-state fraction passes roughly 10⁻⁵–10⁻², cutting the needed cross section by up to 16,000×.

The reading

The LUX-ZEPLIN experiment reported one nuclear-recoil candidate near 248 keV and interpreted it as inelastic dark matter scattering endothermically, $\chi_1 N \to \chi_2 N$, from a halo made only of the ground state $\chi_1$. This paper argues that in pseudo-Dirac models the excited state $\chi_2$ can also survive to the present day, and that once it does the reverse exothermic transition $\chi_2 N \to \chi_1 N$ takes over: it dominates whenever the excited-state fraction $f$ exceeds roughly $10^{-5}$–$10^{-2}$ depending on the splitting, and at $f = 1/2$ it lowers the cross section required to explain the candidate by factors of 80 to $1.6\times10^{4}$ for $\delta = 250$–$350$ keV while removing the endothermic kinematic cutoff. Because the published LZ intervals assume a pure $\chi_1$ halo, they do not apply to the two-state case. The author rebuilds LZ's statistical inference from public projections, validates the recast against the published intervals, and shows that a thermal benchmark ($m_\chi \sim 1$ TeV, $\delta \sim 297$ keV) which explains the event with $\chi_1$ alone is pushed a factor of 30 above the 90% C.L. upper limit when half the halo is $\chi_2$.

What carries the argument

The working object is the two-component scattering rate (Eq. 2) built on the pair of minimum-speed functions $v^\pm_{\rm min} = |m_A E_R/\mu_{\chi A} \pm \delta|/\sqrt{2m_A E_R}$, where the plus sign encodes the endothermic threshold and the minus sign the exothermic one; the exothermic branch reaches $v^-_{\rm min} = 0$ at $E^\star_R = \mu_{\chi A}\delta/m_A$, which is what lets the whole halo contribute. Around that point the signal becomes nearly independent of $\delta$, the required cross section grows approximately linearly with $m_{\chi_1}$ once kinematics stops restricting the signal, and above a critical fraction ($6\times10^{-3}$ at 250 keV, $1.4\times10^{-3}$ at 300 keV, $3\times10^{-5}$ at 350 keV) the required cross section scales as $1/f$. The recast itself is a binned Poisson likelihood (Eq. 5) built from the public projection of LZ Fig. 5, using the shell-model xenon response of Refs. [6,7] corrected to LZ's spectra by a ratio $C(E_R)$ taken at $\delta=0$, with the HEPData event list as data.

Extended reading notes

Core claim

The central claim is that the published LUX-ZEPLIN interpretation of the 248 keV candidate in terms of endothermic inelastic dark matter from a pure $\chi_1$ halo does not cover the realistic pseudo-Dirac case in which both mass eigenstates survive. When both states are present, the scattering rate is the sum of an endothermic and an exothermic term, and the exothermic term has no velocity threshold: at the recoil energy $E^\star_R = \mu_{\chi A}\delta/m_A$ the minimum speed vanishes, so the entire halo rather than only its high-velocity tail scatters. As a result the exothermic channel dominates as soon as the excited-state fraction exceeds $f \gtrsim 10^{-5}$–$10^{-2}$ depending on $\delta$; at $f=1/2$ the cross section needed to explain the candidate drops by factors of 80 to $1.6\times10^{4}$ for $\delta = 250$–$350$ keV, the endothermic kinematic cutoff $\delta_{\rm max}$ disappears with the allowed region extending to $\delta = 500$ keV, and a published benchmark that fits the event with $\chi_1$ alone lies a factor of 30 above the 90% C.L. upper limit.

Load-bearing premise

The whole two-state scenario requires the excited state $\chi_2$ to survive in the halo today, meaning its lifetime must exceed the age of the Universe; the author states this bound explicitly in the Section 4 footnote, notes it is model dependent and not included, and if $\chi_2$ decays the fraction $f$ goes to zero and the exothermic dominance, the cross-section reductions, and the benchmark exclusion all vanish.

Editorial extensions

If this is right

  • Direct-detection analyses of inelastic dark matter must treat the halo as two-component once any excited-state population survives; ignoring $\chi_2$ misstates the required cross section by orders of magnitude.
  • Above the critical fraction the required cross section scales as $1/f$, so exclusion limits and best-fit regions of single-state analyses shift by the same factor.
  • The endothermic kinematic cutoff $\delta_{\rm max} = \tfrac12\mu_{\chi A}(v_{\rm esc}+|v_E|)^2$ no longer bounds the signal, and the allowed region extends to $\delta = 500$ keV for $f=1/2$.
  • The thermal pseudo-Dirac benchmark of Ref. [9] ($m_\chi \simeq 1$ TeV, $\delta \simeq 297$ keV, $\sigma_N \simeq 6.5\times10^{-43}$ cm²), which fits the event with $\chi_1$ alone, is excluded when half the halo is $\chi_2$.
  • The recast framework, validated against the published LZ intervals within 26% for $\delta \ge 50$ keV, supplies a reusable binned likelihood for other two-state signals.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The same two-state logic transfers to other targets: an argon detector, as raised in a related paper, could discriminate the exothermic peak at $E^\star_R$ from the endothermic high-energy tail because the two spectra peak at different recoil energies.
  • Because even a sub-percent excited fraction shifts the result, published single-state upper limits on $\sigma_N$ should be read as applying only to the $f \approx 0$ corner of the two-state parameter space; the paper's own phrasing is that a subdominant excited-state population cannot in general be neglected.
  • A direct external test is the $\chi_2$ lifetime: searches for a monochromatic $\gamma$ line from $\chi_2 \to \chi_1\gamma$ or the opening of $\chi_2 \to \chi_1 e^+e^-$ above $\delta \approx 1$ MeV would settle whether $f$ is actually non-zero today.
  • If LZ's two-dimensional signal and background densities were made public, the unbinned likelihood could be rebuilt directly, which would confirm or correct the author's finding that binning lowers the local significance by 0.2–0.7σ and delays the lower-limit onset by about 20 keV.
Original abstract

The LUX-ZEPLIN (LZ) experiment has reported a single nuclear-recoil candidate at $E_R \simeq 248$keV. LZ interprets it in terms of inelastic dark matter that scatters endothermically from its ground state, $\chi_1 N \to \chi_2 N$. In pseudo-Dirac models the excited state $\chi_2$ can survive to the present day, and exothermic scattering, $\chi_2 N \to \chi_1 N$, is then also open. The published LZ intervals do not describe this case. We rebuild the LZ statistical inference and validate it against the published intervals. We then apply it to a halo that contains a fraction $f$ of $\chi_2$. For $f \gtrsim 10^{-5}$--$10^{-2}$, depending on the mass splitting, the exothermic contribution dominates. For $f=1/2$ the cross section required by the candidate drops by factors of $80$ to $1.6\times10^4$ for $\delta = 250$--$350$~keV, and the kinematic cutoff of the endothermic signal disappears. A benchmark that explains the event with $\chi_1$ alone lies a factor of $30$ above the upper limit when half of the halo is $\chi_2$.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-29: 1 request, 7,013 input tokens.

What the abstract itself states, as probabilities: claims first 0.08 · states limitation 0.20 · human participants 0.02 · controlled comparison 0.26 · randomized assignment 0.01 · states quantitative result 0.98 · compares with prior results 0.93 · mentions code or data availability 0.03.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

The LUX-ZEPLIN (LZ) experiment has reported a single nuclear-recoil candidate at $E_R \simeq 248$keV. LZ interprets it in terms of inelastic dark matter that scatters endothermically from its ground state, $\chi_1 N \to \chi_2 N$. In pseudo-Dirac models the excited state $\chi_2$ can survive to the present day, and exothermic scattering, $\chi_2 N \to \chi_1 N$, is then also open. The published LZ intervals do not describe this case. We rebuild the LZ statistical inference and validate it against the published intervals. We then apply it to a halo that contains a fraction $f$ of $\chi_2$. For $f \gtrsim 10^{-5}$--$10^{-2}$, depending on the mass splitting, the exothermic contribution dominates. For $f=1/2$ the cross section required by the candidate drops by factors of $80$ to $1.6\times10^4$ for $\delta = 250$--$350$~keV, and the kinematic cutoff of the endothermic signal disappears. A benchmark that explains the event with $\chi_1$ alone lies a factor of $30$ above the upper limit when half of the halo is $\chi_2$.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 The LUX-ZEPLIN (LZ) experiment has reported a single nuclear-recoil candidate at $E_R \simeq 248$keV. background 0.85 0.12 0.03 asserted 1.00
s1 LZ interprets it in terms of inelastic dark matter that scatters endothermically from its ground state, $\chi_1 N \to \chi_2 N$. background 0.81 0.07 0.04 asserted 1.00
s2 In pseudo-Dirac models the excited state $\chi_2$ can survive to the present day, and exothermic scattering, $\chi_2 N \to \chi_1 N$, is then also open. background 0.98 0.07 0.05 hedged possibility 0.71
s3 The published LZ intervals do not describe this case. gap 0.97 0.10 0.08 asserted 1.00
s4 We rebuild the LZ statistical inference and validate it against the published intervals. method 0.84 0.94 0.03 asserted 0.98
s5 We then apply it to a halo that contains a fraction $f$ of $\chi_2$. method 0.77 0.90 0.05 asserted 0.91
s6 For $f \gtrsim 10^{-5}$--$10^{-2}$, depending on the mass splitting, the exothermic contribution dominates. result 1.00 0.88 0.27 asserted 0.99
s7 For $f=1/2$ the cross section required by the candidate drops by factors of $80$ to $1.6\times10^4$ for $\delta = 250$--$350$~keV, and the kinematic cutoff of the endothermic signal disappears. result 0.99 0.89 0.31 asserted 1.00
s8 A benchmark that explains the event with $\chi_1$ alone lies a factor of $30$ above the upper limit when half of the halo is $\chi_2$. result 0.85 0.83 0.33 asserted 1.00

What kind of paper the classifier reports this is: theoretical result 0.76 · evidence from analysis of existing data 0.90.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. 90% C.L. intervals for endothermic O s 1. Black: LZ data release [58], with the ±1σ and ±2σ sensitivity bands. Red: this work, circles mark the best fit. ε(ER) dR/dER, of Eq. (2). The slices are defined in S1c, so we convert their edges to recoil energy using the constant-energy contours of
Figure 2. 90% C.L. intervals for χ1 only (f = 0, blue, dashed) and χ1 + χ2 with equal abundances (f = 1/2, red, solid). Circles mark the best fit. The dotted line is the endothermic cutoff, Eq. (4). The dotted line is the kinematic endpoint of the endothermic signal, Eq. (4). The blue region diverges earlier, because near the endpoint the recoils lie at E ⋆ R = µχAδ/mA, above the LZ window for mχ1 ≳ 1 TeV. 0 10−4 10−3 10−2 0.1 0.5 1 fraction of χ2 in the halo, f 10−46 10−45 10−44 10−43 10−42 10−41 10−40 10−39 σ N [cm 2 ] δ = 250 keV δ = 300 keV δ = 350 keV
Figure 3. Best fit (lines) and 90% C.L. interval (bands) on σN versus the fraction f of χ2 in the halo, at mχ1 = 1 TeV. The leftmost point is f = 0. tails of the LZ implementation not described in Ref. [4], such as the treatment of nuisance parameters, the grid in the coupling, or a reporting convention of the release. The release also has no entry at δ = 100 keV, and its upper limits differ from those of Fig. S7 by up to 13%. 4 Results The two-component signal enters the same likelihood. Only Fk changes, since Pkj , bkj and nkj describe the detector and the data

Figures reproduced from arXiv:2609.35650 by the authors.

References

Discussion

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