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arxiv: 2605.03630 · v1 · submitted 2026-05-05 · 🌌 astro-ph.HE · hep-ph

Primordial high energy neutrinos

Nicolas Grimbaum Yamamoto This is my paper

Pith reviewed 2026-05-07 14:12 UTC · model grok-4.3

classification 🌌 astro-ph.HE hep-ph
keywords primordial neutrinoshigh energy neutrinoslong-lived relicsBBN constraintsCMB constraintsneutrino telescopesMonte Carlo simulationKM3-230213A
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The pith

Long-lived relics could produce high-energy neutrinos today with sharp spectral features detectable by telescopes.

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

The paper examines primordial high energy neutrinos, called Phenus, generated from the decay or annihilation of long-lived relics before or around recombination. These particles would reach Earth now with energies in the GeV to PeV range and produce fluxes with sharp spectral features. Cosmological bounds from big bang nucleosynthesis and the cosmic microwave background restrict the allowed masses, lifetimes, and decay channels of the relics. A Monte Carlo code is developed to track how final state radiation and interactions with the cosmic neutrino background modify the arriving spectrum. The same code is used to test whether the KM3-230213A ultrahigh-energy event could have a primordial rather than astrophysical origin.

Core claim

Primordial high energy neutrinos from the decay or annihilation of long-lived relics before or around recombination arrive today with GeV-PeV energies and exhibit sharp spectral features. The parameter space for such relics is bounded by BBN and CMB constraints, yet viable regions exist that current and future neutrino telescopes could probe. A dedicated Monte Carlo code calculates the spectral distortion caused by final state radiation and cosmic neutrino background interactions during propagation and is applied to assess a primordial origin for the KM3-230213A event.

What carries the argument

The dedicated Monte Carlo code that computes distortions to the Phenu spectrum from final state radiation and interactions with the cosmic neutrino background.

If this is right

  • Sharp spectral features would distinguish Phenus from conventional astrophysical neutrinos.
  • BBN and CMB data already exclude large portions of the relic mass-lifetime-decay parameter space.
  • Remaining viable regions lie within the sensitivity reach of present and planned neutrino telescopes.
  • The Monte Carlo code supplies a quantitative tool for checking whether any future high-energy event has a primordial origin.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Observation of Phenus would give a direct probe of particle lifetimes and decays at early-universe temperatures inaccessible to other messengers.
  • The same propagation framework could be adapted to assess primordial contributions to other cosmic-ray or gamma-ray signals.
  • Non-observation at next-generation detectors would tighten bounds on long-lived relics beyond current cosmological limits.
  • Compatibility of the KM3-230213A event with a primordial spectrum would motivate dedicated searches for similar events with the predicted features.

Load-bearing premise

Long-lived relics exist with masses, lifetimes and decay channels that yield observable GeV-PeV neutrinos without violating BBN or CMB limits, and the Monte Carlo code correctly captures all relevant propagation physics.

What would settle it

A high-statistics measurement of the high-energy neutrino spectrum that either reveals or rules out the predicted sharp features, or a Monte Carlo run showing that the KM3-230213A event properties are incompatible with any primordial propagation history.

Figures

Figures reproduced from arXiv: 2605.03630 by Nicolas Grimbaum Yamamoto.

Figure 1
Figure 1. Figure 1: Neutrino flux as a function of the observed energy today for the 4 types of sharp view at source ↗
Figure 2
Figure 2. Figure 2: Left: Ratio of the maximal fP allowed by the CMB, BBN and ∆Neff constraints to the one allowed by experimental constraints, in the (mP , τP ) plane. The light grey region corresponds to parameters giving Emax ν < 5 MeV, difficult to probe experimentally. The dashed black line delimits the region where the produced Phenu undergoes on average less than one elastic scattering with a CνB neutrino on its way to… view at source ↗
read the original abstract

Among the few ways to probe the early Universe, neutrinos offer a particular window on high energy phenomena occurring before recombination. We discuss the opportunities of observing primordial high energy neutrinos (Phenus): neutrinos produced before or around recombination from the decay or annihilation of long-lived relics, arriving at detectors today with energies in the GeV-PeV range. We summarise the results of a general study of this scenario, covering the sharp spectral features such fluxes would display, the theoretical (BBN and CMB) and experimental constraints on the source particle parameter space, and the regions that could realistically be probed by current and future neutrino telescopes. We also present a dedicated Monte Carlo code for computing the distortion of the Phenu spectrum by final state radiation and interactions with the cosmic neutrino background during propagation, and apply it to assess the primordial origin hypothesis for the KM3-230213A ultrahigh energy neutrino event.

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

1 major / 1 minor

Summary. The paper discusses opportunities for observing primordial high energy neutrinos (Phenus) produced before or around recombination from decay or annihilation of long-lived relics, arriving today with GeV-PeV energies. It summarizes a general study of sharp spectral features, BBN/CMB and experimental constraints on source particle parameter space, and regions probeable by current/future neutrino telescopes. It presents a dedicated Monte Carlo code to model Phenu spectrum distortions from final state radiation and cosmic neutrino background interactions during propagation, and applies the code to assess the primordial origin hypothesis for the KM3-230213A ultrahigh energy neutrino event.

Significance. If the central claims hold, this work opens a new window on early-Universe high-energy phenomena via neutrinos, complementing BBN and CMB probes. The dedicated Monte Carlo code for propagation effects and its application to a specific observed event (KM3-230213A) are concrete strengths that ground the discussion in data and could guide future telescope searches. The use of external standard BBN/CMB results reduces circularity risk.

major comments (1)
  1. [§4] §4 (Monte Carlo code section): The description of the dedicated MC code lacks explicit validation against analytical limits or existing neutrino propagation codes, as well as details on error handling and any post-hoc selections in the simulation runs. This is load-bearing for the reliability of the assessment of the KM3-230213A event's primordial origin.
minor comments (1)
  1. [Abstract] The abstract and introduction could more explicitly state the assumed ranges for relic mass, lifetime, and branching ratios explored in the parameter-space study.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for the constructive comment on the Monte Carlo code. We address the point below and will revise the manuscript to incorporate additional details that strengthen the presentation.

read point-by-point responses

  1. Referee: [§4] §4 (Monte Carlo code section): The description of the dedicated MC code lacks explicit validation against analytical limits or existing neutrino propagation codes, as well as details on error handling and any post-hoc selections in the simulation runs. This is load-bearing for the reliability of the assessment of the KM3-230213A event's primordial origin.

    Authors: We agree that more explicit validation and implementation details will improve the clarity and robustness of §4. The current manuscript introduces the dedicated Monte Carlo code and outlines its treatment of final-state radiation and cosmic neutrino background interactions, but does not include direct comparisons to analytical limits or other codes. In the revised version we will add a dedicated subsection that (i) shows code outputs against analytical expectations for limiting cases (e.g., pure final-state radiation with no propagation or negligible interactions), (ii) provides benchmark comparisons to existing neutrino propagation frameworks where the physics overlaps, (iii) specifies the error-handling procedure (including event statistics, convergence criteria, and uncertainty estimation), and (iv) states that no post-hoc selections were applied beyond the physical modeling itself. These additions will be placed in §4 and will directly support the reliability of the KM3-230213A analysis without changing the reported results or conclusions. revision: yes

Circularity Check

0 steps flagged

No significant circularity

full rationale

The paper summarizes a parameter-space study of primordial high-energy neutrinos from long-lived relics, grounding constraints in external standard BBN and CMB results rather than internal fits. It introduces a new Monte Carlo code to model propagation distortions (final-state radiation and cosmic neutrino background interactions) and applies the code to an independently observed event (KM3-230213A). No load-bearing step reduces by construction to a fitted parameter, self-defined quantity, or self-citation chain; the derivation chain remains self-contained against external benchmarks and data.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The scenario rests on the domain assumption that long-lived relics exist with appropriate properties; no explicit free parameters or invented entities are detailed in the abstract though the parameter space exploration implies relic mass lifetime and branching ratios are varied subject to external constraints.

axioms (1)
  • domain assumption Standard BBN and CMB constraints can be directly applied to limit the source particle parameter space for relic decays and annihilations
    Invoked in the abstract to constrain the parameter space of long-lived relics.

pith-pipeline@v0.9.0 · 5438 in / 1687 out tokens · 89114 ms · 2026-05-07T14:12:26.872150+00:00 · methodology

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Reference graph

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