REVIEW
Endothermic Dark Matter at LZ from a Decaying Parent
High Energy Physics - Phenomenology · Submitted 29 Sep 2026 · 120 references
Pith's verdict: A clean, honest decay-source realization of the boosted endothermic flux that Ref. [98] left open, though the 248.5 keV peak is an imposed threshold alignment the authors themselves admit they cannot enforce.
T0 · AI review
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Referee report
The paper
The paper claims
A long-lived dark parent decaying to energetic ground-state particles can supply the boosted flux behind LZ's 248 keV recoil candidate, with one event fixing the parent lifetime at about $4.4 \times 10^{20}$ seconds.
The reading
What carries the argument
The central object is the two-body decay $X \to \chi_1 \chi_1$ of a long-lived parent whose rest-frame injection energy $E_0 = m_X/2$ is tuned to the endothermic threshold $E_{1,\text{th}}$ of the upscattering $\chi_1 A \to \chi_2 A$ ($m_2 > m_1 > 0$). Three ingredients carry the argument: (i) the Galactic flux normalization $\Phi_G = 2 f_X D / (m_X \tau_X)$, with the column-density factor $D = 1.61 \times 10^{22}\,\text{GeV}\,\text{cm}^{-2}$ from a smooth NFW halo, which encodes the parent lifetime's role in setting the absolute rate; (ii) the Doppler-broadened incident energy distribution $F_G(E_1)$ (standard deviation $\sigma_E = 0.167$ MeV), which allows the upper part of the line to scatter even when $E_0$ equals a reference nucleus's threshold; and (iii) exact scattering kinematics with natural xenon isotopes and a Helm form factor, which place the efficiency-weighted recoil mode at 248.5 keV. The load-bearing result is Eq. (16), $\tau_X \simeq 4.37 \times 10^{20}\,\text{s}\, f_X[\sigma_{\text{ref}}/(10^{-36}\,\text{cm}^2)]$, which converts one accepted event into a lifetime–cross-section product.
Extended reading notes
Core claim
The paper's central claim is that a long-lived parent $X$ decaying to two energetic ground-state dark particles, followed by endothermic upscattering $\chi_1 A \to \chi_2 A$ with $m_2 > m_1$, is a calculable realization of the boosted flux that the LZ 248 keV recoil candidate would require. For a scalar contact benchmark at $(m_1, m_2, m_X) = (10, 252.6408, 505.8027)$ MeV with the parent rest-frame injection energy aligned to the threshold ($E_0 = m_X/2 = E_{1,\text{th}} = 252.9013$ MeV), the Doppler-broadened incident distribution ($\sigma_E = 0.167$ MeV) combined with exact kinematics over natural xenon isotopes and the LZ efficiency yields an accepted recoil spectrum peaking at 248.5 keV. Requiring one event in the 225–271 keV true-recoil window fixes the product $\Phi_G \sigma_{\text{ref}} = 1.46 \times 10^{-34}\,\text{s}^{-1}$, equivalently $\tau_X \simeq 4.37 \times 10^{20}\,\text{s}\, f_X[\sigma_{\text{ref}}/(10^{-36}\,\text{cm}^2)]$ (Eq. 16). The efficiency-weighted window cross section $\sigma_W \simeq 1.7 \times 10^{-2}\sigma_{\text{ref}}$ gives $\Phi_G \sigma_W \simeq 2.4 \times 10^{-36}\,\text{s}^{-1}$, matching the source-independent target of the earlier analysis this work extends.
Load-bearing premise
The whole peak rests on the parent mass being tuned to within one Doppler line width of the endothermic scattering threshold ($\delta m_X/m_X \lesssim 10^{-3}$, i.e. $|E_0 - E_{1,\text{th}}| \lesssim \sigma_E$), and the paper proposes no symmetry that would make that alignment natural; if the alignment is not natural, the recoil mode falls to 235.5 keV and the low-energy leakage $R_{\text{low}}$ rises to 0.062.
Editorial extensions
If this is right
- Dark decay, not annihilation, can supply the source: at the benchmark masses the decay flux ($\Phi_G = 146\,\text{cm}^{-2}\text{s}^{-1}$ at $\tau_X = 4.37 \times 10^{20}\,\text{s}$, $f_X = 1$) exceeds a reference annihilation flux with $\langle\sigma v\rangle = 3 \times 10^{-26}\,\text{cm}^3\text{s}^{-1}$ by $4.6 \times 10^4$, reaching the required normalization through the lifetime rather than th
- The required parent is long-lived but not cosmologically stable: $\tau_X \simeq 4.4 \times 10^{20}\,\text{s}$ means only about $0.1\%$ of the parent decays over the age of the Universe ($t_U/\tau_X \simeq 1.0 \times 10^{-3}$), while the CMB limit on decays into dark radiation ($\tau \gtrsim 5 \times 10^{18}\,\text{s}$) bounds $\sigma_{\text{ref}} \gtrsim 10^{-38}\,\text{cm}^2$ at $f_X = 1$.
- The same incident distribution yields quantitative target discriminants: the fraction of the flux above threshold is $P_{^{131}\text{Xe}} = 0.50$, $P_{^{184}\text{W}} = 0.67$, $P_{^{72}\text{Ge}} = 0.10$, $P_{^{40}\text{Ar}} = 1.9 \times 10^{-5}$, and $P_{^{28}\text{Si}} = 0$, so extended xenon windows and heavier targets test the source while argon data sharply separate it from a cold exothermic
- A companion elastic operator must be suppressed to $\sigma_{11}/\sigma_{\text{ref}} \lesssim 1.2 \times 10^{-5}$ (an amplitude ratio below $3.5 \times 10^{-3}$, Eq. 17) to keep additional low-energy events below 0.1 at $N_W = 1$; the parity that would forbid it also forbids the portal coupling and the invisible width, framing the suppression as a stated design target rather than a symmetry-enforce
Reading between the lines
- The threshold alignment $\delta m_X/m_X \lesssim 10^{-3}$ is put in by hand — the paper explicitly states no symmetry enforces it — so the 248.5 keV peak is currently a fit rather than a derived consequence; a completion that generates the mass relation naturally would materially strengthen the scenario, and one that does not leaves it tuned.
- Because the $c_A$ sensitivity shifts the required flux–cross-section product by a factor 0.3–5, the precision of the quoted lifetime is limited chiefly by the xenon nuclear response near the diffraction minimum; an independent re-evaluation of the Helm form factor at these recoil energies would shrink that uncertainty.
- The daughters radiate as dark radiation until $z \sim 25$ and cannot penetrate the light-nucleus overburden endothermically (optical depth $\sim 10^{-10}$), so the signal cannot be regenerated near the detector — a feature that makes the scenario strictly a source-driven prediction testable only through added exposure and heavier targets.
- The elastic suppression, the portal coupling, and the invisible width are governed by the same symmetry breaking, so any ultraviolet completion must satisfy accelerating, cosmological, and Eq. (17) constraints jointly; quantifying that joint fit is exactly the detector-level and ultraviolet-specific work the authors flag as remaining.
Original abstract
The LUX-ZEPLIN experiment has reported a nuclear-recoil candidate near 248 keV. We investigate a decay source for an endothermic interpretation: a long-lived dark parent produces energetic ground-state particles that upscatter on xenon. The parent mass fixes the injection energy, while its lifetime determines the absolute flux through the Galactic dark-matter column density. We include the Doppler broadening from parent and observer motion and use exact scattering kinematics with natural xenon isotopes. For a scalar contact interaction, a benchmark with a 10 MeV incident state and a 506 MeV parent gives an efficiency-weighted recoil spectrum peaked at 248.5 keV. One event in a specified high-energy window requires $\tau_X \simeq 4.4 \times 10^{20}$ s $f_X [\sigma_{\text{ref}}/(10^{-36} \text{cm}^2)]$, with a normalization sensitive to the nuclear response. We quantify the accompanying elastic-scattering requirement and the target-dependent thresholds. The calculation identifies a decay realization of the boosted flux and the portal conditions needed to preserve the recoil signal; a detector-level fit and ultraviolet-specific constraints remain necessary for a complete experimental assessment.
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 LUX-ZEPLIN experiment has reported a nuclear-recoil candidate near 248 keV. We investigate a decay source for an endothermic interpretation: a long-lived dark parent produces energetic ground-state particles that upscatter on xenon. The parent mass fixes the injection energy, while its lifetime determines the absolute flux through the Galactic dark-matter column density. We include the Doppler broadening from parent and observer motion and use exact scattering kinematics with natural xenon isotopes. For a scalar contact interaction, a benchmark with a 10 MeV incident state and a 506 MeV parent gives an efficiency-weighted recoil spectrum peaked at 248.5 keV. One event in a specified high-energy window requires $\tau_X \simeq 4.4 \times 10^{20}$ s $f_X [\sigma_{\text{ref}}/(10^{-36} \text{cm}^2)]$, with a normalization sensitive to the nuclear response. We quantify the accompanying elastic-scattering requirement and the target-dependent thresholds. The calculation identifies a decay realization of the boosted flux and the portal conditions needed to preserve the recoil signal; a detector-level fit and ultraviolet-specific constraints remain necessary for a complete experimental assessment.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The LUX-ZEPLIN experiment has reported a nuclear-recoil candidate near 248 keV. | background 0.99 | 0.12 | 0.05 | asserted 0.99 |
| s1 | We investigate a decay source for an endothermic interpretation: a long-lived dark parent produces energetic ground-state particles that upscatter on xenon. | contribution 0.93 | 0.55 | 0.10 | asserted 0.50 |
| s2 | The parent mass fixes the injection energy, while its lifetime determines the absolute flux through the Galactic dark-matter column density. | method 0.74 | 0.21 | 0.05 | asserted 1.00 |
| s3 | We include the Doppler broadening from parent and observer motion and use exact scattering kinematics with natural xenon isotopes. | method 1.00 | 0.89 | 0.02 | asserted 0.97 |
| s4 | For a scalar contact interaction, a benchmark with a 10 MeV incident state and a 506 MeV parent gives an efficiency-weighted recoil spectrum peaked at 248.5 keV. | result 0.99 | 0.88 | 0.16 | asserted 0.99 |
| s5 | One event in a specified high-energy window requires $\tau_X \simeq 4.4 \times 10^{20}$ s $f_X [\sigma_{\text{ref}}/(10^{-36} \text{cm}^2)]$, with a normalization sensitive to the nuclear response. | result 1.00 | 0.84 | 0.14 | asserted 0.99 |
| s6 | We quantify the accompanying elastic-scattering requirement and the target-dependent thresholds. | contribution 0.65 | 0.83 | 0.04 | asserted 0.93 |
| s7 | The calculation identifies a decay realization of the boosted flux and the portal conditions needed to preserve the recoil signal; a detector-level fit and ultraviolet-specific constraints remain necessary for a complete experimental assessment. | limitation 0.66 | 0.79 | 0.44 | asserted 0.92 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:2609.37545 by the authors.
References
Discussion
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