Limits on Primordial Black Hole Evaporation from LUX-ZEPLIN

Jitumani Kalita

REVIEW

Limits on Primordial Black Hole Evaporation from LUX-ZEPLIN

Jitumani Kalita

Cosmology and Nongalactic Astrophysics · Submitted 24 Sep 2026

Pith's verdict: The qualitative no-go on Hawking-boosted DM explaining the LZ 248 keV event is sound; the 'strict theorem' and the definite 10^-6 bound overreach because m_chi is never specified or scanned.

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arXiv ID2609.29462 (version 2)

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CitationJitumani Kalita. "Limits on Primordial Black Hole Evaporation from LUX-ZEPLIN." arXiv:2609.29462 (2026). https://pith.science/paper/BB5JE3UR

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Editor's note

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The paper's real contribution is the no-go: a thermal Hawking spectrum cannot explain the LZ 248 keV event, because the flux reaching 123 MeV (needed for that recoil) is Boltzmann-suppressed by roughly exp(-5.8), while the 17-55 MeV population near the flux peak inevitably overproduces 5-50 keV recoils in a window where LZ saw nothing. That argument is right, and the specific application to the LZ anomaly, plus the inversion into an f_PBH limit across the 2x10^11-2x10^13 g window, is new — the underlying overproduction physics is familiar from cosmic-ray boosted DM (Bringmann-Pospelov, Cappiello et al.), but the mapping onto the PBH-evaporation case is not in the prior literature. The low-to-high recoil ratio is coupling-independent, so the qualitative no-go survives the unspecified couplings.

Where it overreaches. First, the 'strict No-Go theorem' is actually a no-go for an unexamined mass window. The whole calculation is written in the ultra-relativistic limit — Eq. (3) and Eq. (10) both drop m_chi — but the dark matter mass is never specified or scanned. If m_chi sits at 10-20 MeV, the recoil kinematics shift; if it exceeds roughly the 21 MeV Hawking temperature, the relevant emission is Boltzmann-suppressed and the mechanism dies. The paper calls this a strict theorem when it holds within a mass window the paper never declares. Second, the headline f_PBH < 10^-6 is underdetermined: N_0 scales as lambda^2, so the bound is a function of the unspecified coupling, mediator scale, and mass, yet it is presented as a definite number. Third, Eq. (10) is asserted without derivation — minor, since it is a standard result from the cited inelastic boosted DM papers — and the NREFT extension to relativistic incident particles, with full A^2 coherence at a momentum transfer sitting on the second Helm diffraction lobe, is unjustified. Fourth, the limit is not benchmarked against the gamma-ray and positron exclusions the paper itself cites (Laha's 511 keV line, Dasgupta et al.), which for a limit-setting paper is a genuine omission.

None of this kills the qualitative conclusion; the 248 keV event is very hard for Hawking-boosted DM to explain. But the paper packages a model-contingent calculation as a strict theorem plus a definite bound, and it never states its own mass window. The referee process would force that to the surface. I would send this to review — the no-go is clean, citable, and useful to people working on PBH evaporation constraints or boosted-DM interpretations of LZ, and the fixes are straightforward: scan m_chi, state the window, soften the quantitative claims, and add the comparison against existing astrophysical bounds.

Referee report

Significance

The elastic-scattering arm of the argument is conceptually sound and genuinely parameter-free in the coupling: the ratio of the low-recoil rate (fed by E_χ ≈ 18–55 MeV near the Hawking peak) to the 248 keV rate (fed by E_χ ≳ 123 MeV, Boltzmann-suppressed by exp(−5.8)) does not depend on λ or Λ, so the qualitative conclusion that a thermal boosted flux overproduces 5.4–50 keV recoils is robust within its stated kinematic limit. The proposal to convert this into a direct-detection bound on f_PBH is a legitimate and potentially interesting new channel. However, the result as submitted has three load-bearing gaps: (i) the 'strict' no-go and the quoted f_PBH ≲ 10^-6 bound are only valid for an unstated and un-scanned dark-matter mass window (m_χ ≲ 5–10 MeV genuinely ultra-relativistic at the relevant incident energies); (ii) the inelastic analysis is internally inconsistent with the paper's own Eq.

Major objections (4)

[§III.A and §II, Eqs. (2)–(3) and (10)] The 'strict No-Go theorem' (b84) and the quoted bound f_PBH ≲ 10^-6 both rest on an unexamined dark-matter mass window. Eq. (3) is the m_χ → 0 limit of Eq. (2), and Eq. (10) likewise drops m_χ, so the entire calculation is ultra-relativistic in the incident particle at the energies that source the low-recoil window (E_χ ≈ 18–55 MeV). If m_χ ≈ 10–20 MeV, the kinematics at those energies must be recomputed from Eq. (2), and if m_χ ≳ 21 MeV the emission into the 18–55 MeV band is itself Boltzmann-suppressed. The paper never states a mass requirement beyond 'light dark matter' and never scans m_χ. The elastic ratio argument is robust only under an explicit condition m_χ ≲ 5–10 MeV, which must be stated as a condition on the theorem rather than left implicit.
[§III.B, Eq. (10), Fig. 1] The inelastic analysis is internally inconsistent with Eq. (10). For δ = 220 keV and m_N = 122.3 GeV, Eq. (10) gives E_min^χ ≈ 397 MeV at E_R = 5.4 keV, decaying to ≈ 205 MeV at 50 keV, with a global minimum ≈ 164 MeV near E_R ≈ 110 keV and ≈ 239 MeV at 248 keV. Low-energy recoils therefore require incident energies comparable to or much larger than the 248 keV recoil, and the 5.4–50 keV window is fed by E_χ ≈ 205–400 MeV — the tail, not the thermal peak. The text's justification that 'the energy cost of the mass splitting is negligible, i.e., E_χ ≫ δ' (b66) uses the wrong controlling criterion; what matters is δ relative to E_R, amplified by (m_N/2E_R)^{1/2}. The claim (b68) that the inelastic curve 'outstrips the 248 keV event rate by over three orders of magnitude' needs to be re-derived; on the face of Eq. (10) the inelastic channel suppresses the low-recoil window relative to elastic scattering rather than leaving it unsuppressed. The solid indigo curves in Figs. 1 and 2, and the b101 claim that the inelastic constraint is more stringent at low mass, are unsupported until this is recomputed.
[§IV, Eqs. (11)–(13), Fig. 2] The headline result 'exclude f_PBH ≳ 10^-6 at 90% CL' is not a definite number as stated. Since dσ/dE_R ∝ λ² (2 m_N E_R/Λ²)^n, N_0 in Eq. (12) is proportional to λ²/Λ^{2n}, so f_max = N_LZ_limit/N_0 scales as Λ^{2n}/λ². The dimensionless coupling λ and mediator scale Λ are never assigned benchmark values, and the dark-matter mass m_χ is likewise unspecified; a smaller λ erases the bound. In addition, the numerical value of N_LZ_limit — the 90% CL upper limit on new-physics events in the 5.4–50 keV window for the 2.84 tonne-year exposure — is never quoted, although Eq. (13) is exactly this ratio. Without stated benchmarks for (λ, Λ, m_χ) and for N_LZ_limit, the vertical scale of Fig. 2 and the b5/b114 headline cannot be reproduced or compared with other PBH constraints.
[§III.A, Eq. (9)] The NREFT form (q/Λ)^{2n} with n = 1 and n = 3 is used for relativistic incident particles with momenta q ≈ 246 MeV, but the operator basis of Refs. [62,63] is derived in the non-relativistic limit and is not automatically valid at p_χ ≈ E_χ with E_χ up to several hundred MeV. Since Λ is unspecified (it could be below q, in which case the expansion breaks down), the cross-section parameterization is an unvalidated extrapolation. The elastic no-go ratio is insensitive to this only to the extent that both the low- and high-recoil rates share the same (q/Λ)^{2n} structure, which should be the case asserted explicitly.

Minor objections (5)

[Abstract and §IV] The inequality direction in the statement of the bound is inconsistent: the abstract and b5 say 'excluding f_PBH ≳ 10^-6' while §IV and b95/b114 say 'f_PBH ≲ 10^-6'. This should be corrected to one consistent form.
[§III and Fig. 1] The claimed quantitative excesses — 'roughly two orders of magnitude' for the q^6 elastic peak (b71) and 'over three orders of magnitude' for the inelastic case — are asserted as results of numerical integration but no numbers are reported. For reproducibility, the integrated ratios of the 5.4–50 keV rate to the 248 keV rate should be quoted for each scenario.
[§IV, Eq. (11)] The detection-efficiency function ε(E_R) and the LZ limit value N_LZ_limit are referred to Ref. [17] but never specified, so Eq. (11) cannot be evaluated by a reader. The paper should state the efficiency curve used and the actual 90% CL limit adopted.
[§III, Eqs. (7)–(8)] The normalization of N_0 also depends on the Milky Way D-factor D_MW (Eq. 7) and on the spin/statistics of the emitted dark-matter particle entering d²Nχ/dtdEχ, neither of which is stated. For a paper whose final bound is a number, these inputs should be listed.
[§V] The conclusion that LZ 'closes parameter space that is traditionally difficult to constrain via standard gamma-ray or positron astrophysical bounds' needs a caveat: existing constraints from evaporation products (e.g., Refs. [40,49,70]) already exclude parts of the covered low-mass region, and the model additionally presupposes a beyond-Standard-Model dark-matter species with xenon couplings. The scope of the no-go/bound is conditional on that particle existing.

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The paper

The paper claims

A strict No-Go rules out Hawking-boosted dark matter from evaporating primordial black holes as the source of the LZ 248 keV anomaly, and inverts the low-energy null into f_PBH ≲ 10^-6 limits.

The reading

The LUX-ZEPLIN detector recently recorded a single nuclear recoil at 248 ± 23 keV that standard cold dark matter cannot produce, alongside a strict null result in its 5.4–50 keV search window. This paper asks whether the anomalous event could come from light dark matter freshly boiled off evaporating primordial black holes in the $10^{11}$–$10^{13}$ g mass range. The author argues it cannot: the thermal Hawking spectrum inevitably emits far more particles in the ~17.5 MeV range, which produce 5–50 keV recoils, than the ~123 MeV particles needed for a 248 keV recoil, so any model tuned to the high-energy event overproduces events where LZ sees none. That failure is turned around into a new probe: the low-energy null places f_PBH ≲ $10^{-6}$ at M_PBH ~ 2×$10^{11}$ g at 90% confidence, among the strongest constraints in this mass window.

What carries the argument

The argument runs through the differential recoil rate dR/dE_R, a convolution of the Hawking-boosted flux (Galactic and extragalactic components, both governed by the geometrical-optics emission spectrum with its exp(−E_χ/T_PBH) Boltzmann tail) with an NREFT cross section carrying an explicit momentum dependence (2 m_N E_R / Λ²)^n and the Helm nuclear form factor that loses coherence on the second diffraction lobe at the ~246 MeV/c transfer of the anomalous event. The decisive quantity is the separation between threshold energies: E_χ^min ≈ 123 MeV for a 248 keV recoil versus ≈ 17.5 MeV for a 5.4 keV recoil, which places the low-energy events squarely at the thermal peak of a 5×$10^{11}$ g black hole (T_PBH ≈ 21 MeV) while the high-energy event sits exponentially far out on the tail.

Extended reading notes

Core claim

The paper's central claim is a strict No-Go theorem: the LZ 248 keV anomaly cannot be explained by a contemporary flux of Hawking-boosted dark matter from evaporating primordial black holes, and this holds whether the nuclear scattering is modeled by elastic momentum-dependent NREFT operators up to $q^{6}$ scaling or by endothermic inelastic mass transitions with a splitting near δ = 220 keV. The mechanism is the exponential Boltzmann suppression of the Hawking spectrum: producing a 5 keV recoil needs an incident particle near the 21 MeV peak of the flux, while producing 248 keV needs E_χ ≳ 123 MeV, a phase-space penalty of roughly exp(−5.8) that no operator or inelastic truncation overcomes. Because the low-energy excess is inevitable, the author inverts the LZ null result in the 5.4–50 keV window into upper limits on the PBH dark-matter fraction, excluding f_PBH ≳ $10^{-6}$ at M_PBH ~ 2×$10^{11}$ g at 90% confidence and stating that in the low-mass regime the inelastic constraint is the stronger of the two.

Load-bearing premise

The entire constraint assumes a light dark-matter species with unspecified mass and an effective coupling to xenon nuclei; if that coupling is small, the predicted event rate shrinks and the f_PBH bound disappears.

Editorial extensions

If this is right

If the No-Go theorem is correct, the 248 keV LZ event is not a Hawking-boosted dark matter signal, ruling out evaporating PBHs as its source.
The LZ 5.4–50 keV null window becomes a competitive probe of light PBHs, excluding f_PBH down to ~10^-6 at M_PBH ~ 2×10^11 g.
At low PBH mass the endothermic inelastic constraint (δ = 220 keV) is more stringent than the extreme q^6 elastic one, a reversal from the standard halo case.
Sensitivity to PBHs fades for M_PBH ≳ 10^13 g, where the Hawking temperature drops below the ~17.5 MeV threshold needed to excite even a 5.4 keV recoil.
Direct-detection experiments built for halo WIMPs double as detectors for the relativistic dark sector emitted by evaporating black holes.

Reading between the lines

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

Because the limit scales as f_max = N_limit / N_0 with N_0 ∝ λ², the quoted f_PBH bound is really a joint constraint on f_PBH times the dark-matter–nucleon coupling squared; an arbitrarily small λ erases the limit.
The dark-matter mass m_χ is never fixed, and the whole calculation assumes m_χ ≪ E_χ; a species with m_χ near or above the ~21 MeV Hawking temperature would reshuffle the low-energy tail and could evade the spectral argument.
The monochromatic PBH mass spectrum is load-bearing; an extended mass distribution near 10^13 g would smear the sharp loss of sensitivity the paper derives at that boundary.
The f_PBH bound does not depend on the 248 keV event being a real signal: it rests on the 5.4–50 keV null, so it survives even if the anomaly turns out to be a background fluctuation.

Original abstract

The LUX-ZEPLIN (LZ) collaboration recently reported a singular anomalous nuclear recoil event at $248 \pm 23 \, \rm keV$, accompanied by a strict null result for unexplained excesses in their primary $5.4-50 \, \rm keV$ search window. The macroscopic momentum transfer required to generate this event is kinematically inaccessible to standard halo cold dark matter (CDM), naturally motivating models featuring light, relativistic relics. In this work, we investigate whether a contemporary flux of Hawking-boosted dark matter (DM) emitted by evaporating Primordial Black Holes (PBHs) in the $10^{11}-10^{13} \, \rm g $ mass range can source the anomaly. By parameterizing the nuclear scattering with heavy non-relativistic effective field theory (NREFT) operators and endothermic inelastic mass transitions, we formulate a strict kinematic exclusion. We demonstrate that the inherent thermal nature of the Hawking emission, coupled with the relativistic kinematics of the incident flux, inevitably overproduces low-energy recoils, aggressively violating the LZ background bounds. Utilizing this failure, we map the LZ low-energy null results into novel, highly stringent upper limits on the PBH abundance fraction, excluding $f_{\rm PBH} \gtrsim 10^{-6}$ for $M_{\rm PBH} \sim 2 \times 10^{11} \, \rm g $ at $ 90 \% $ confidence level.

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, 2026-09-25: 1 request, 5,396 input tokens.

What the abstract itself states, as probabilities: claims first 0.05 · states limitation 0.46 · human participants 0.02 · controlled comparison 0.17 · randomized assignment 0.01 · states quantitative result 0.98 · compares with prior results 0.30 · mentions code or data availability 0.02.

The abstract, sentence by sentence

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

The LUX-ZEPLIN (LZ) collaboration recently reported a singular anomalous nuclear recoil event at $248 \pm 23 \; \rm keV$, accompanied by a strict null result for unexplained excesses in their primary $5.4-50 \; \rm keV$ search window. The macroscopic momentum transfer required to generate this event is kinematically inaccessible to standard halo cold dark matter, naturally motivating models featuring light, relativistic relics. In this work, we investigate whether a contemporary flux of Hawking-boosted dark matter emitted by evaporating Primordial Black Holes (PBHs) in the $10^{11}-10^{13} \; \rm g $ mass range can source the anomaly. By parameterizing the nuclear scattering with heavy non-relativistic effective field theory (NREFT) operators and endothermic inelastic mass transitions, we formulate a strict kinematic exclusion. We demonstrate that the inherent thermal nature of the Hawking emission, coupled with the relativistic kinematics of the incident flux, inevitably overproduces low-energy recoils, aggressively violating the LZ background bounds. Utilizing this failure, we map the LZ low-energy null results into novel, highly stringent upper limits on the PBH abundance fraction, excluding $f_{\rm PBH} \gtrsim 10^{-6}$ for $M_{\rm PBH} \sim 2 \times 10^{11} \; \rm g $ at 90\% confidence level.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	The LUX-ZEPLIN (LZ) collaboration recently reported a singular anomalous nuclear recoil event at $248 \pm 23 \; \rm keV$, accompanied by a strict null result for unexplained excesses in their primary $5.4-50 \; \rm keV$ search window.	background 0.98	0.16	0.02	asserted 1.00
s1	The macroscopic momentum transfer required to generate this event is kinematically inaccessible to standard halo cold dark matter, naturally motivating models featuring light, relativistic relics.	background 0.62	0.10	0.02	asserted 0.81
s2	In this work, we investigate whether a contemporary flux of Hawking-boosted dark matter emitted by evaporating Primordial Black Holes (PBHs) in the $10^{11}-10^{13} \; \rm g $ mass range can source the anomaly.	contribution 0.98	0.63	0.04	hedged possibility 0.74
s3	By parameterizing the nuclear scattering with heavy non-relativistic effective field theory (NREFT) operators and endothermic inelastic mass transitions, we formulate a strict kinematic exclusion.	method 0.97	0.92	0.06	asserted 1.00
s4	We demonstrate that the inherent thermal nature of the Hawking emission, coupled with the relativistic kinematics of the incident flux, inevitably overproduces low-energy recoils, aggressively violating the LZ background bounds.	result 0.92	0.86	0.23	asserted 1.00
s5	Utilizing this failure, we map the LZ low-energy null results into novel, highly stringent upper limits on the PBH abundance fraction, excluding $f_{\rm PBH} \gtrsim 10^{-6}$ for $M_{\rm PBH} \sim 2 \times 10^{11} \; \rm g $ at 90\% confidence level.	result 1.00	0.95	0.89	asserted 1.00

What kind of paper the classifier reports this is: theoretical result 1.00 · evidence from analysis of existing data 0.68.

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.** FIG. 1. Normalized differential recoil spectra for Hawking

**Figure 2.** FIG. 2. Upper bounds (90% C.L.) on the primordial black

Figures reproduced from arXiv:2609.29462 by the authors.

References

Forward citations

Cited by 4 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

The Electron-Recoil Response Tail and the LZ High-Energy Event Background
hep-ex 2026-10 accept novelty 7.0 of 10

The published LZ electron-recoil response characterization does not determine the tail probability needed for the background estimate; tiny quantile shifts (0.14%) can change the ER contribution by a factor of 13.
Testing the inelastic dark matter interpretation of the LZ event with heavy-element experiments
hep-ph 2026-10 conditional novelty 6.0 of 10

The thermal Higgsino dark matter model can fit the LZ 248 keV event only if the dark matter velocity distribution has an extreme high-velocity tail, and heavy-element experiments can test this scenario.
The LZ inelastic splitting as a probe of anomaly mediation
hep-ph 2026-10 conditional novelty 6.0 of 10

An MRSSM with R-symmetric Dirac bino, wino coannihilation, N = 2 high-scale coupling relations, and warped anomaly mediation can realize the LZ 248 keV inelastic dark matter candidate with spectra from 300 to 850 GeV.
One Event, Two Frontiers: From LZ to LHC
hep-ph 2026-10 conditional novelty 5.0 of 10

Most elastic dark matter interactions proposed to explain the LZ 248 keV event are excluded by LHC monojet data for dark matter masses below roughly a TeV, assuming EFT validity.

Discussion

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