REVIEW
Cosmological Constrained Axion-Portal Inelastic Dark Matter for the LZ Event
High Energy Physics - Phenomenology · Submitted 15 Sep 2026 · 83 references
Pith's verdict: A solid inelastic-DM paper with a genuinely new cosmological twist; the central claim holds up, but the kinetic-decoupling assumption deserves scrutiny in refereeing.
T0 · AI review
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Referee report
The paper
The paper claims
This paper establishes that the present-day abundance of excited dark-matter particles in a two-state inelastic model is set by the early-universe conversion process χ2χ2↔χ1χ1, and that this fraction depends critically on whether the…
The reading
What carries the argument
The central object is the exchange symmetry E (χ1↔χ2, a→a) which enforce pure off-diagonal couplings and equal diagonal masses. The boltzmann equation for f2 integrates the conversion cross section ⟨σv⟩_{22→11}, which depends on the Lorentz structure of the DM–ALP interation, and on the ALP propagator which retains full momentum dependence because the typical momentum transfer q_* ≜ 0.8 GeΓ is comparable to m_.
Extended reading notes
Core claim
The excited-state fraction f2 of the heavier dark-atter state χ2 is not fixed by hand but emerges from the Boltzmann evolution of χ2χ2↔χ1χ1 in the early universe. For scalar DM–ALP couplings (Γ =1,1/2) the conversion cross section is large, so f2 today iss ≊$01^{-7}$–$10^{-5}$; for pseudoscar couplings (Γ =ιγ5,ιγ5/2) the cross section is suppressed, leaving f2 ≈) 1/2. Thus the direction of inelastic scattering—endothermic χ1N→χ2N vs. exothermic χ2N→χ1N—is dynamically selected by the early-universe history of the dark sector.
Load-bearing premise
The exchange symmetry E (χ1↔χ2, a→a) is assumed exact, which ensures the two states are nearly degenerate and the coupling to the ALP is purely off-diagonal; any breaking of this symmetry would introduce a diagonal mass difference and additional diagonal couplings that could shift the splitting and the conversion dynamics.
Editorial extensions
If this is right
- The recoi spectra of the LZ candidate event can be explained by either endothermic (scalar) or exothermic (pseudoscalar) inelastic scattering, both peaking near 250 keV.
- Anual modulation is predicted to be large (≈100% fractional amplitude) for endothermic scenarios and negligible for exothermic scenarios, allowing direct experimental discrimination.
- The ALP portal (g→g→gg + aF→F) can be tested at future collider experiments such as DUNE, FASER, and HL-LHC, as shown in the paper's constraint plot.
- The three-body decay χ2→χ1γγ is slow enough to avoid cosmological and X-ray bounds for the parameter region relevant to the LZ event, especially for the small f2 of the scalar scenarios.
- If the LZ candidate is real, the combined exposure of LZ, PandaX-4T, and XENONnT may already contain ∼5 such events, enabling a statistical test within a few years.
Reading between the lines
- Because the excited-state fraction is set by cosmology, it is not free: future experiments that measure the absolute event rate and the annual modulation can independently determine f2 and test whether it matches the predicted value for a given coupling type.
- The small f2 in scalar scenarios means that exothermic scattering can still contribute noticeably if the mass splitting forces endothermic scattering to rely on the high-velocity tail of the halo. This creates a mixed signature that may help pin down both the coupling structure and the splitting.
- The model postulates that the ALP couples to a→G→G and a→F→F with comparable strength. If collider searches for long-lived ALPs fail to find any signal at the projected sensitivities, the scenario may be constrained independently of direct-detection data.
- The mechanism linking early-universe conversion to the sign of inelastic scattering could be applied to other dark-sector portals, not only axion-like particles, as long as the mediator has a mass comparable to the typical momentum transfer.
Original abstract
The recent high-recoil candidate event reported by LUX-ZEPLIN (LZ) motivates dark-matter scenarios with nonstandard kinematics and momentum-dependent interactions. We study a two-state inelastic dark-matter model in which $\chi_1$ and $\chi_2$ couple off-diagonally to an axion-like particle (ALP) that also couples to gluons and photons. Unlike treatments that assume only one dark-matter state is present today, we track the cosmological evolution of both states. The excited state $\chi_2$ is sufficiently long-lived to survive to the present, with its relic fraction determined by the dark-sector conversion process $\chi_2\chi_2\leftrightarrow\chi_1\chi_1$. We find that this fraction depends strongly on the Lorentz structure of the DM--ALP interaction: scalar transition couplings efficiently deplete $\chi_2$, favoring endothermic $\chi_1 N\to\chi_2 N$ scattering, whereas pseudoscalar transition couplings preserve $f_2\simeq1/2$, yielding recoil spectra dominated by exothermic $\chi_2 N\to\chi_1 N$ down-scattering. Benchmark spectra in all four scenarios can peak near the observed recoil energy of $250\,\mathrm{keV}$. Our results demonstrate that the dominant direction of inelastic scattering in direct detection can be dynamically selected by the early-Universe evolution of the dark sector. With additional data, annual modulation measurements could distinguish these scenarios. We further show that the ALP portal can be directly probed at colliders.
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.
The abstract, sentence by sentence
The recent high-recoil candidate event reported by LUX-ZEPLIN (LZ) motivates dark-matter scenarios with nonstandard kinematics and momentum-dependent interactions. We study a two-state inelastic dark-matter model in which $\chi_1$ and $\chi_2$ couple off-diagonally to an axion-like particle (ALP) that also couples to gluons and photons. Unlike treatments that assume only one dark-matter state is present today, we track the cosmological evolution of both states. The excited state $\chi_2$ is sufficiently long-lived to survive to the present, with its relic fraction determined by the dark-sector conversion process $\chi_2\chi_2\leftrightarrow\chi_1\chi_1$. We find that this fraction depends strongly on the Lorentz structure of the DM--ALP interaction: scalar transition couplings efficiently deplete $\chi_2$, favoring endothermic $\chi_1 N\to\chi_2 N$ scattering, whereas pseudoscalar transition couplings preserve $f_2\simeq1/2$, yielding recoil spectra dominated by exothermic $\chi_2 N\to\chi_1 N$ down-scattering. Benchmark spectra in all four scenarios can peak near the observed recoil energy of $250\,\mathrm{keV}$. Our results demonstrate that the dominant direction of inelastic scattering in direct detection can be dynamically selected by the early-Universe evolution of the dark sector. With additional data, annual modulation measurements could distinguish these scenarios. We further show that the ALP portal can be directly probed at colliders.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The recent high-recoil candidate event reported by LUX-ZEPLIN (LZ) motivates dark-matter scenarios with nonstandard kinematics and momentum-dependent interactions. | background 1.00 | 0.05 | 0.04 | asserted 0.72 |
| s1 | We study a two-state inelastic dark-matter model in which $\chi_1$ and $\chi_2$ couple off-diagonally to an axion-like particle (ALP) that also couples to gluons and photons. | contribution 0.65 | 0.62 | 0.03 | asserted 0.97 |
| s2 | Unlike treatments that assume only one dark-matter state is present today, we track the cosmological evolution of both states. | contribution 0.52 | 0.80 | 0.03 | asserted 0.98 |
| s3 | The excited state $\chi_2$ is sufficiently long-lived to survive to the present, with its relic fraction determined by the dark-sector conversion process $\chi_2\chi_2\leftrightarrow\chi_1\chi_1$. | method 0.50 | 0.51 | 0.06 | asserted 1.00 |
| s4 | We find that this fraction depends strongly on the Lorentz structure of the DM--ALP interaction: scalar transition couplings efficiently deplete $\chi_2$, favoring endothermic $\chi_1 N\to\chi_2 N$ scattering, whereas pseudoscalar transition couplings preserve $f_2\simeq1/2$, yielding recoil spectra dominated by exothermic $\chi_2 N\to\chi_1 N$ down-scattering. | result 0.99 | 0.95 | 0.39 | asserted 1.00 |
| s5 | Benchmark spectra in all four scenarios can peak near the observed recoil energy of $250\,\mathrm{keV}$. | result 1.00 | 0.90 | 0.08 | asserted 0.68 |
| s6 | Our results demonstrate that the dominant direction of inelastic scattering in direct detection can be dynamically selected by the early-Universe evolution of the dark sector. | interpretation 0.93 | 0.91 | 0.80 | asserted 1.00 |
| s7 | With additional data, annual modulation measurements could distinguish these scenarios. | future work 0.98 | 0.18 | 0.04 | hedged possibility 1.00 |
| s8 | We further show that the ALP portal can be directly probed at colliders. | result 0.65 | 0.85 | 0.05 | asserted 0.99 |
What was examined, and what was not
- arXiv listing abstract v1
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
Figures
Figures reproduced from arXiv:2609.17412 by the authors.
References
Discussion
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