REVIEW
Caught by its own light: Wimpzillas, the LHAASO knee and the diffuse $\gamma$-ray sky
High Energy Astrophysical Phenomena · Submitted 15 Sep 2026 · 66 references
Pith's verdict: Clean multi-messenger ruling-out of decaying superheavy dark matter as the origin of the LHAASO knee, with careful systematics and honest limitations.
Why Pith says this
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Referee report
The paper
The paper claims
A superheavy dark matter decay can reproduce the LHAASO proton knee, but it emits three times more energy in photons than in nucleons — and measured photons rule it out by one to three orders of magnitude.
The reading
What carries the argument
The load-bearing object is the fixed energy ratio between the photon and nucleon yields of a single fragmentation cascade, $f_\gamma/f_N \simeq 2.9$: an fragmenting quark jet produces mostly pions, about a third of them neutral, and each neutral pion decays promptly into two photons, while baryon production is suppressed by the cost of pulling a diquark pair from the vacuum. This ratio varies only between 2.3 and 3.6 across decay channels and stays nearly flat over twelve decades of parent mass, so any model tuned to put PeV nucleons in the LHAASO window must simultaneously put about three times that energy into PeV photons. Around this ratio the argument is carried by two further pieces: a diffusion calculation in a slab halo that converts halo decays into the local nucleon flux through an effective confinement time $\tau_\mathrm{eff} = \psi_\odot/K$, and a reconstruction of LHAASO's two-step background-subtraction chain for the diffuse Galactic emission, which removes most of an extended halo component, leaving a few per cent of its raw column in the inner window. The KASCADE-Grande photon-fraction limit supplies the subtraction-free cross-check.
Extended reading notes
Core claim
On the paper's own terms, the central discovery is a self-consistency failure. The same decaying relic that can account for the LHAASO proton spectrum — mass $m_\chi \simeq 5\times 10^{7}$ GeV, lifetime $\tau_\chi \simeq 2.5\times 10^{25}$ s, decaying into a parton cascade in the halo — necessarily produces a photon flux that violates two independent measurements: the LHAASO diffuse Galactic emission (exceeded by hundreds of times on the raw halo intensity, and by about a factor of twenty even after the analysis's two-step background subtraction is applied to the prediction) and the KASCADE-Grande limit on the photon fraction of cosmic radiation above 3.7 PeV (exceeded by a factor of 60 to 900). Because hadronisation delivers about three times as much energy to photons as to nucleons, almost independently of decay channel and parent mass, none of the six channels considered escapes. The paper therefore concludes that superheavy dark matter decay cannot be the source of the knee, while simultaneously reporting a new direct lower bound on the relic lifetime, $\tau_\chi \gtrsim 5\times 10^{25}$ s near $m_\chi \simeq 7\times 10^{8}$ GeV, obtained from the LHAASO proton data alone in a mass range previously left empty by hadronic cosmic-ray probes.
Load-bearing premise
The load-bearing premise is the paper's reconstruction of LHAASO's background-subtraction chain: that the direct-integration and 20-degree-smoothing steps reduce an extended halo signal to a few per cent of its raw column in the inner Galactic-plane window. If the real chain removes far less of the halo, the factor-of-twenty photon exclusion could shrink toward borderline, although the KASCADE-Grande exclusion, which needs no background model, would stand.
Editorial extensions
If this is right
- Decaying superheavy dark matter cannot be the source of the LHAASO knee: the lifetime the proton fit demands (about $2.5\times 10^{25}$ s) sits roughly twenty times below the photon lower bound from LHAASO diffuse emission after the background chain, and 60 to 900 times below the KASCADE-Grande photon-fraction bound.
- The exclusion survives every variation the authors tested — three hadronic-interaction-model reconstructions, four propagation setups, and six decay channels — so the hardening and knee call for an astrophysical explanation such as PeVatron sources or a transport transition rather than new physics.
- The LHAASO proton spectrum on its own yields a direct lifetime bound $\tau_\chi \gtrsim 5\times 10^{25}$ s near $m_\chi \simeq 7\times 10^{8}$ GeV, the first hadronic cosmic-ray constraint in the $10^{2}$ to $10^{9}$ GeV mass window that had been left uncovered between antiproton and ultra-high-energy proton measurements.
- A halo-filling source maintains about 35 times more local nucleon density per unit source rate than a disc-confined source, so dark matter decay signals are underestimated whenever disc-calibrated confinement times are imported; the paper also notes that the halo picture predicts a nucleon dipole toward the Galactic centre that a dedicated anisotropy analysis could test.
Reading between the lines
- Any future claim that a parton cascade produces a measured nucleon feature — from decaying superheavy dark matter, evaporating primordial black holes, or similar exotic sources — carries a same-cascade photon flux of comparable or larger energy, so the diffuse gamma-ray sky is a generic, often stronger cross-check that should be applied before such a fit is announced.
- Because the LHAASO diffuse Galactic emission analysis is constructed to subtract large-scale structure, limits on halo-filling signals derived from it are systematically optimistic; a dedicated search that treats the halo as signal in the background regions too would be the stricter probe, as the paper's raw-flux benchmark of several hundred already suggests.
- The paper's own caveat points to the decisive next measurement: the proton systematic covariance is unpublished, and if its correlation length is large the spectral resemblance to a decay would carry little statistical weight, so publishing the covariance matrix would settle how seriously the nucleon feature should be taken.
- The KASCADE-Grande exclusion is bracketed by two irreducible unknowns — the median-versus-threshold energy assignment and the detection efficiency for spectra harder than the $E^{-2}$ calibration — so a dedicated efficiency simulation for a decaying-dark-matter spectrum could pin the exclusion factor inside its 60 to 900 range.
Original abstract
We use the proton spectrum resolved by LHAASO across the knee to test superheavy dark matter in the nucleon channel at PeV energies. Both the hardening at $0.34$ PeV and the knee at $3.3$ PeV are reproduced by nucleons from a relic of mass $5\cdot 10^{7}$ GeV decaying in the Galactic halo, with the mass as the only shape parameter of the spectrum and the background index profiled under an independent prior on the proton slope below $0.1$ PeV. It works equally well in all three hadronic-interaction-model reconstructions. On the other hand, the photons of the same cascade rule out this interpretation by about two orders of magnitude against the photon fraction of the cosmic radiation, and by about one order of magnitude against the LHAASO diffuse Galactic emission.
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
We use the proton spectrum resolved by LHAASO across the knee to test superheavy dark matter in the nucleon channel at PeV energies. Both the hardening at $0.34$ PeV and the knee at $3.3$ PeV are reproduced by nucleons from a relic of mass $5\cdot 10^{7}$ GeV decaying in the Galactic halo, with the mass as the only shape parameter of the spectrum and the background index profiled under an independent prior on the proton slope below $0.1$ PeV. It works equally well in all three hadronic-interaction-model reconstructions. On the other hand, the photons of the same cascade rule out this interpretation by about two orders of magnitude against the photon fraction of the cosmic radiation, and by about one order of magnitude against the LHAASO diffuse Galactic emission.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We use the proton spectrum resolved by LHAASO across the knee to test superheavy dark matter in the nucleon channel at PeV energies. | contribution 0.61 | 0.87 | 0.08 | asserted 0.83 |
| s1 | Both the hardening at $0.34$ PeV and the knee at $3.3$ PeV are reproduced by nucleons from a relic of mass $5\cdot 10^{7}$ GeV decaying in the Galactic halo, with the mass as the only shape parameter of the spectrum and the background index profiled under an independent prior on the proton slope below $0.1$ PeV. | result 0.96 | 0.89 | 0.61 | asserted 1.00 |
| s2 | It works equally well in all three hadronic-interaction-model reconstructions. | result 0.98 | 0.85 | 0.08 | asserted 1.00 |
| s3 | On the other hand, the photons of the same cascade rule out this interpretation by about two orders of magnitude against the photon fraction of the cosmic radiation, and by about one order of magnitude against the LHAASO diffuse Galactic emission. | result 0.79 | 0.77 | 0.51 | asserted 0.99 |
What was examined, and what was not
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References
Discussion
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