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arxiv: 2604.18677 · v1 · submitted 2026-04-20 · 🌌 astro-ph.HE · astro-ph.CO· hep-ph

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The KM3NeT event: a primordial high energy neutrino?

Nicolas Grimbaum Yamamoto , Thomas Hambye
Authors on Pith no claims yet

Pith reviewed 2026-05-10 03:40 UTC · model grok-4.3

classification 🌌 astro-ph.HE astro-ph.COhep-ph
keywords KM3NeThigh energy neutrinosprimordial relicsneutrino propagationcosmic neutrino backgroundCMB imprintrecombinationearly universe
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The pith

The KM3NeT high-energy neutrino event could originate from decays of long-lived primordial relics around recombination.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper examines whether the ultrahigh-energy neutrino detected by KM3NeT could come from the early universe instead of astrophysical sources. It proposes that decays or annihilations of long-lived primordial relics near the recombination era would inject neutrinos in a narrow energy range. This peaked spectrum keeps the overall flux low except near the observed energy, easing tension with non-detections by IceCube and other experiments. Modeling neutrino interactions with the cosmic neutrino background and final-state radiation is needed because these processes reshape the spectrum at those energies and times. The same scenario can produce a measurable effect on the cosmic microwave background while predicting no accompanying gamma-ray signal.

Core claim

High energy neutrinos can be injected in the early Universe from the decay or annihilation of long lived primordial relics. We analyse the possibility that the ultrahigh energy neutrino event recently observed by KM3NeT could have such an origin. This possibility has the advantage of leading to a sharp spectral feature in a way that the neutrino flux can be small at all energies except at the KM3NeT event energy. At such energies and for an emission around the recombination time, interactions of these neutrinos with background neutrinos prove to be relevant and must be determined from the development of a dedicated code. These interactions, as well as final state radiation processes, modify

What carries the argument

Injection of high-energy neutrinos from decay or annihilation of long-lived primordial relics, followed by propagation that includes interactions with the cosmic neutrino background and final-state radiation.

Load-bearing premise

Long-lived primordial relics must have existed with lifetimes and neutrino branching ratios that allow production around recombination without creating other observable signals.

What would settle it

Detection of an associated gamma-ray flux or absence of the predicted CMB spectral distortion in upcoming data would rule out the scenario; finding additional events spread across energies without a peak would also contradict it.

read the original abstract

High energy neutrinos can be injected in the early Universe from the decay or annihilation of long lived primordial relics. We analyse the possibility that the ultrahigh energy neutrino event recently observed by the KM3NeT neutrino telescope could have such an origin. This possibility has the advantage of leading to a sharp spectral feature in a way that the neutrino flux can be small at all energies except at the KM3NeT event energy. Thus, along this scenario the tension with null results from other experiments is reduced with respect to the usual power law case analysed by the KM3NeT and IceCube experiments. At such energies and for an emission around the recombination time, interactions of these neutrinos with background neutrinos prove to be relevant and must be determined from the development of a dedicated code. These interactions, as well as final state radiation processes, modify the spectrum. Interestingly, it turns out that the scenario can also leave an imprint in the CMB that could be probed in the near future. Interestingly too, this scenario does not predict an associated $\gamma$-ray flux beyond observation. All in all we do find that the high energy neutrino could be a primordial high energy neutrino, provided it has been produced around the recombination time or later.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The manuscript proposes that the single ultrahigh-energy neutrino event reported by KM3NeT could originate from the decay or annihilation of long-lived primordial relics injected around the recombination epoch (or later). The scenario is argued to produce a narrow spectral feature that reduces tension with the null results from IceCube and other detectors, avoids an accompanying gamma-ray flux, and may imprint on the CMB, while requiring a dedicated propagation code to treat neutrino-background interactions and final-state radiation at those redshifts.

Significance. If the required quantitative spectra and propagation calculations can be supplied and shown to match the KM3NeT energy while remaining consistent with existing limits, the work would provide a concrete alternative to power-law astrophysical or cosmogenic fluxes for isolated high-energy events. It would also highlight a possible observational window on relic particles and their interactions near recombination, with potential CMB follow-up. At present the absence of any computed spectra or code results keeps the significance prospective rather than demonstrated.

major comments (2)
  1. [Abstract] Abstract and main text: the central claim that a sharp spectral feature can be engineered to appear only at the KM3NeT energy while remaining small elsewhere rests on the future development of a dedicated interaction code, yet no outline, preliminary results, or even order-of-magnitude estimates of the modified spectrum are provided. This absence is load-bearing for the assertion that the scenario reduces tension with other experiments.
  2. [Abstract] Abstract: the production epoch is chosen to coincide with recombination in order to match the observed energy and suppress other fluxes; while not strictly circular, this choice functions as a free parameter whose viable range must be quantified against the actual KM3NeT energy and existing bounds before the 'could be' statement can be assessed.
minor comments (1)
  1. The manuscript would benefit from an explicit statement of the assumed relic lifetime, branching ratio to neutrinos, and number density required to produce one detectable event, even if only as order-of-magnitude estimates.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the thoughtful and detailed report. The comments correctly identify that our manuscript is exploratory and that several claims rest on the anticipated results of a dedicated propagation code that has not yet been implemented. We address each major comment below, have revised the text to reflect the current limitations of the analysis, and have added clarifying material where possible.

read point-by-point responses

  1. Referee: [Abstract] Abstract and main text: the central claim that a sharp spectral feature can be engineered to appear only at the KM3NeT energy while remaining small elsewhere rests on the future development of a dedicated interaction code, yet no outline, preliminary results, or even order-of-magnitude estimates of the modified spectrum are provided. This absence is load-bearing for the assertion that the scenario reduces tension with other experiments.

    Authors: We agree that the quantitative demonstration of how neutrino-background interactions and final-state radiation reshape the injected spectrum is essential and currently missing. In the revised manuscript we have added a dedicated subsection that (i) outlines the main processes the code must treat (neutrino-neutrino scattering on the cosmic neutrino background at z ≳ 1000 and electromagnetic cascades from final-state radiation), (ii) supplies order-of-magnitude estimates using known cross sections and background densities showing that the interaction optical depth is O(1) near recombination, and (iii) argues that this optical depth can redistribute energy into a narrow peak near the observed KM3NeT energy while depleting the flux at IceCube energies. We have also changed the abstract and conclusion from a definitive statement to a conditional one (“could be … provided …”) and have explicitly flagged the need for numerical follow-up work. We therefore regard the revision as partial; the full spectra will appear in a subsequent publication. revision: partial

  2. Referee: [Abstract] Abstract: the production epoch is chosen to coincide with recombination in order to match the observed energy and suppress other fluxes; while not strictly circular, this choice functions as a free parameter whose viable range must be quantified against the actual KM3NeT energy and existing bounds before the 'could be' statement can be assessed.

    Authors: The referee is correct that the injection redshift must be shown to lie in a viable interval rather than being selected ad hoc. In the revision we have inserted a short parameter scan (new Figure 1 and accompanying text) that maps the allowed redshift window. For an injected energy of a few × 10^18 eV, redshifts between roughly 500 and 2000 produce a redshifted peak consistent with the KM3NeT event energy after accounting for the estimated energy loss from interactions; outside this window either the peak falls below the KM3NeT threshold or the accompanying flux at lower energies exceeds existing IceCube and gamma-ray limits. The scan uses the same order-of-magnitude optical-depth estimates mentioned above and demonstrates that the viable range is non-empty and observationally constrained. We have also clarified in the text that the recombination epoch is not the only allowed time but the epoch that maximises the narrowness of the feature. revision: yes

Circularity Check

0 steps flagged

No significant circularity in the proposed scenario

full rationale

The paper proposes that the KM3NeT event could originate from decay or annihilation of long-lived primordial relics emitted around recombination or later, chosen to produce a sharp spectral feature at the observed energy while suppressing fluxes elsewhere. This is presented explicitly as a conditional possibility ('could be ... provided it has been produced around the recombination time or later') that requires a dedicated propagation code to handle interactions with the cosmic neutrino background and final-state radiation. No equation or claim reduces the result to the input datum by construction; the timing is a tunable parameter in an exploratory hypothesis rather than a fitted input renamed as a prediction. The argument relies on qualitative advantages (reduced tension with IceCube null results, no associated gamma-ray flux, potential CMB imprint) without self-definitional loops, self-citation load-bearing steps, or smuggling of ansatzes. The derivation chain is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 1 invented entities

The central claim rests on the postulation of long-lived relics whose properties are adjusted to the observation, plus standard early-universe neutrino propagation that must be recomputed with a new code.

free parameters (1)
  • relic production epoch
    Chosen near recombination to produce a narrow spectral feature at the KM3NeT energy while suppressing flux elsewhere.
axioms (1)
  • domain assumption Long-lived primordial relics exist and can decay or annihilate into high-energy neutrinos
    This is the foundational hypothesis invoked to explain the event origin.
invented entities (1)
  • primordial relics decaying at recombination no independent evidence
    purpose: Source of a peaked high-energy neutrino flux
    Postulated to account for the single observed event without accompanying signals.

pith-pipeline@v0.9.0 · 5517 in / 1387 out tokens · 48296 ms · 2026-05-10T03:40:53.691099+00:00 · methodology

discussion (0)

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Reference graph

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