KM3-230213A: Ultra-High-Energy Neutrino Event

• KM3-230213A is an ultra-high-energy neutrino event observed by KM3NeT/ARCA with an inferred neutrino energy around 220 PeV and a reconstructed muon energy near 120 PeV.
• The event’s analysis employs multimessenger follow-ups and flux inferences that reveal tensions with IceCube and Auger non-detections while constraining cosmic-ray source evolution.
• KM3-230213A drives innovative tests in fundamental physics, including Lorentz invariance violation and dark-matter decay scenarios, prompting new approaches beyond the Standard Model.

Searching arXiv for papers on KM3-230213A to ground the article in the literature. KM3-230213A is an ultra-high-energy neutrino event reported by the KM3NeT Collaboration and widely treated as the most energetic neutrino observed to date, with an inferred neutrino energy of approximately $220$ PeV and a reconstructed muon energy of $120^{+110}_{-60}\,\mathrm{PeV}$ (Satunin, 13 Feb 2025). It has become a focal object in multimessenger astrophysics because a single event at this energy probes the extreme tail of neutrino production models, the proton content and evolution of ultra-high-energy cosmic-ray source populations, and several beyond-standard-model scenarios, while also exposing a quantitative tension with null results from IceCube and the Pierre Auger Observatory (Myhr, 8 May 2026). The event has therefore been used both as a phenomenological source probe and as a constraint on Lorentz-invariance violation, in-vacuo dispersion, dark-matter decay, and related frameworks (Yang et al., 25 Feb 2025).

1. Discovery, reconstruction, and basic observational status

KM3-230213A was recorded by the 21-line configuration of KM3NeT/ARCA on 2023-02-13 and published in February 2025 (Myhr, 8 May 2026). It was detected as an almost horizontal or near-horizontal track event, with best-fit reconstructed direction $\mathrm{RA}=94.3^\circ$, $\mathrm{Dec}=-7.8^\circ$ and a reported $68\%$ containment radius $R(68\%)=1.5^\circ$; a $90\%$ containment radius of $2.2^\circ$ is quoted in VERITAS follow-up work (Amelino-Camelia et al., 18 Feb 2025). The event is associated with a reconstructed muon energy $E_\mu = 120^{+110}_{-60}\,\mathrm{PeV}$, while the inferred neutrino energy is typically given as $220^{+570}_{-110}\,\mathrm{PeV}$, with a 68% confidence interval of $120^{+110}_{-60}\,\mathrm{PeV}$0 and a 90% confidence interval of $120^{+110}_{-60}\,\mathrm{PeV}$1 (Yang et al., 25 Feb 2025).

The event is treated as astrophysical rather than atmospheric because the amount of material traversed along the reconstructed trajectory strongly disfavors an atmospheric-muon origin. One review states that the path implies roughly $120^{+110}_{-60}\,\mathrm{PeV}$2, making an atmospheric muon explanation “highly unlikely” (Oliveira et al., 22 Jul 2025). A dedicated PBR study similarly argues that atmospheric muons are not viable once the required water-equivalent depth and survival probability are taken into account (Olinto et al., 3 Jul 2025).

Its exceptional status derives not only from absolute energy, but also from comparison with previous neutrino detections. One Lorentz-invariance-violation analysis emphasizes that the event energy is “almost two orders of magnitude above the previous most energetic neutrino seen by IceCube” (Satunin, 13 Feb 2025). Another review describes it as the first detected astrophysical neutrino above $120^{+110}_{-60}\,\mathrm{PeV}$3 PeV and the first direct detection of a neutrino in the ultra-high-energy domain, defined there as $120^{+110}_{-60}\,\mathrm{PeV}$4 (Myhr, 8 May 2026).

2. Flux interpretation and tension with other observatories

KM3-230213A quickly became central to the global ultra-high-energy neutrino landscape because a naive flux inference from a single event appears difficult to reconcile with non-observations elsewhere. One synthesis reports a single-flavor flux normalization

$120^{+110}_{-60}\,\mathrm{PeV}$5

from a joint fit involving KM3NeT, IceCube, and Auger, with a tension of about $120^{+110}_{-60}\,\mathrm{PeV}$6 relative to the null detections by IceCube and Auger (Myhr, 8 May 2026). Another review summarizes a higher event-implied flux level,

$120^{+110}_{-60}\,\mathrm{PeV}$7

and then notes that the discrepancy softens once a joint isotropic fit is used (Oliveira et al., 22 Jul 2025).

Several papers frame this mismatch in terms of source class. Under a diffuse isotropic interpretation, the tension with IceCube is quoted as $120^{+110}_{-60}\,\mathrm{PeV}$8, while transient-source interpretations reduce it to $120^{+110}_{-60}\,\mathrm{PeV}$9 (Olinto et al., 3 Jul 2025). A review gives a similar hierarchy: $\mathrm{RA}=94.3^\circ$0–$\mathrm{RA}=94.3^\circ$1 for diffuse or cosmogenic scenarios, $\mathrm{RA}=94.3^\circ$2 for stationary point sources, and $\mathrm{RA}=94.3^\circ$3 for transient point sources (Oliveira et al., 22 Jul 2025). This has made transience, anisotropy, or detector-geometry dependence recurrent themes in the literature.

That tension is methodologically important because it forces model comparisons to use not just the observed event, but also the absence of comparable events in IceCube and Auger. This is explicit in cosmogenic analyses, GRB-blastwave fits, and several beyond-standard-model papers, all of which treat KM3-230213A not as an isolated datum but as a constraint coupled to wider exposure and null results (Alhebsi et al., 13 Mar 2026).

3. Source classes and astrophysical interpretations

Astrophysical interpretations of KM3-230213A divide broadly into Galactic, extragalactic transient, cosmogenic, and population-level diffuse scenarios. A review states that a Galactic origin is effectively ruled out because counterpart searches found no powerful Galactic emitters and no relevant gamma-ray sources in the error region seen by HAWC or LHAASO (Myhr, 8 May 2026). Another review similarly describes a Galactic origin as highly unlikely (Oliveira et al., 22 Jul 2025).

Extragalactic transient scenarios remain prominent because they reduce the KM3NeT–IceCube tension. In hadronic models, however, a neutrino source should also produce gamma rays, leading to dedicated follow-up studies. A Fermi-LAT cascade search found no convincing GeV counterpart and therefore constrained combinations of source redshift and intergalactic magnetic field, excluding roughly $\mathrm{RA}=94.3^\circ$4 for $\mathrm{RA}=94.3^\circ$5 in its benchmark transient hadronic picture (Crnogorčević et al., 20 Mar 2025). VERITAS follow-up observations likewise found no statistically significant very-high-energy gamma-ray excess and derived a $\mathrm{RA}=94.3^\circ$6 CL upper limit at the event position of

$\mathrm{RA}=94.3^\circ$7

for an assumed spectrum with $\mathrm{RA}=94.3^\circ$8 (Mooney, 29 Sep 2025).

Within the localization region, several papers discuss blazars as candidate counterparts. A review notes that 17 blazars were identified within the uncertainty region, with three showing flaring behavior near the event time, but no conclusive association was established (Myhr, 8 May 2026). A source-model study argues that PMN J0606-0724 is the most plausible counterpart because of a radio flare peaking 5 days after the neutrino, while interpreting the event through a jet–red-giant interaction and shock acceleration scenario (Clairfontaine et al., 3 Nov 2025). In that framework, the baryon injection is attributed to the red giant, but the dominant photon field setting the neutrino energy scale is argued to be colder infrared emission from the dusty torus rather than the red-giant photosphere, since the latter would more naturally yield $\mathrm{RA}=94.3^\circ$9–10 PeV neutrinos instead of $\mathrm{Dec}=-7.8^\circ$0 PeV (Clairfontaine et al., 3 Nov 2025).

Cosmogenic interpretations have also been developed in detail. One study fits a two-population UHECR model and finds that, if the event is interpreted using only KM3NeT exposure, the proton-source evolution must be strongly positive, with $\mathrm{Dec}=-7.8^\circ$1 at 68% CL, while inclusion of Auger and IceCube null detections relaxes this to $\mathrm{Dec}=-7.8^\circ$2 (Alhebsi et al., 13 Mar 2026). Another multimessenger transport calculation argues that a steep sub-ankle proton component is disfavored because it saturates the Fermi-LAT isotropic gamma-ray background, whereas a hard proton spectrum extending beyond $\mathrm{Dec}=-7.8^\circ$3 with evolution $\mathrm{Dec}=-7.8^\circ$4 can reproduce KM3-230213A without violating gamma-ray or neutrino limits, provided the proton fraction is $\mathrm{Dec}=-7.8^\circ$5 at $\mathrm{Dec}=-7.8^\circ$6 (Cermenati et al., 16 Jul 2025). A different cosmogenic-point-source study proposes that a nearby transient cosmogenic-neutrino source with weak intergalactic magnetic field could be more favorable than diffuse cosmogenic emission, particularly because it reduces the contrast with IceCube non-detection (Zhang et al., 14 Apr 2025).

Long-duration GRB blastwaves have likewise been tested as a population explanation. A Poisson-likelihood analysis using the joint exposure of KM3NeT/ARCA, IceCube, and Auger constrains the baryon loading to

$\mathrm{Dec}=-7.8^\circ$7

at $\mathrm{Dec}=-7.8^\circ$8 confidence for ISM densities $\mathrm{Dec}=-7.8^\circ$9, while for wind-like environments it finds

$68\%$0

for $68\%$1 (Collaboration et al., 18 Sep 2025). This suggests that GRB blastwaves remain viable contributors to the diffuse UHE neutrino background, but only in restricted parameter regions (Collaboration et al., 18 Sep 2025).

4. Fundamental-physics tests enabled by the event

Because many Lorentz-violating effects scale steeply with energy, KM3-230213A immediately became a powerful probe of Lorentz invariance in the neutrino sector. One analysis considers superluminal neutrino Lorentz-invariance violation with modified dispersion relation

$68\%$2

for $68\%$3, and uses the absence of neutrino splitting $68\%$4 to derive lower limits on the LIV scale (Satunin, 13 Feb 2025). Under an extragalactic source-distance assumption $68\%$5 Mpc, it finds

$68\%$6

and identifies neutrino splitting, rather than $68\%$7, as the dominant constraint (Satunin, 13 Feb 2025).

A second LIV study adopts an isotropic modified-dispersion framework and propagates neutrino spectra including the vacuum decay channels $68\%$8 and $68\%$9. It concludes that the most conservative acceptable quadratic limit is

$R(68\%)=1.5^\circ$0

while stating explicitly that the existing data do not provide a high-confidence constraint on the linear $R(68\%)=1.5^\circ$1 scale in that setup (Yang et al., 25 Feb 2025). A review further summarizes related velocity-based constraints as $R(68\%)=1.5^\circ$2 (Oliveira et al., 22 Jul 2025).

KM3-230213A has also been used as a case study for in-vacuo dispersion. One paper investigates whether the neutrino could be associated with GRB090401B, observed about 14 years earlier, under a linear delay law

$R(68\%)=1.5^\circ$3

and reports directional overlap, energy consistency, and a Monte Carlo p-value of $R(68\%)=1.5^\circ$4 for the GRB090401B–KM3-230213A pairing (Amelino-Camelia et al., 18 Feb 2025). The authors explicitly state that this is “intriguing but far from conclusive,” and frame the exercise as proof of principle for archival transient searches with decade-scale delays rather than as evidence for quantum gravity (Amelino-Camelia et al., 18 Feb 2025).

5. Dark-matter and other beyond-standard-model interpretations

KM3-230213A has generated an extensive literature on superheavy or very heavy dark matter. The simplest class of models uses two-body dark-matter decay to neutrinos, exploiting the kinematic relation $R(68\%)=1.5^\circ$5. A paper based on a singlet-fermion dark matter extension of type-I seesaw interprets the event through $R(68\%)=1.5^\circ$6, deriving an allowed dark-matter mass range

$R(68\%)=1.5^\circ$7

with corresponding lifetimes

$R(68\%)=1.5^\circ$8

while also predicting stochastic gravitational waves from the disappearance of domain walls formed when a $R(68\%)=1.5^\circ$9 symmetry is spontaneously broken (Kohri et al., 6 Mar 2025).

Another paper proposes a vector dark matter model based on a new $90\%$0 gauge symmetry, with benchmark dark matter mass $90\%$1 PeV and viable lifetime range

$90\%$2

In that model, the same $90\%$3 breaking produces cosmic strings with predicted tension

$90\%$4

which the authors describe as consistent with recent pulsar timing array observations (Su et al., 29 Jul 2025).

More cosmological dark-sector scenarios have also been explored. One study links KM3-230213A and high-energy IceCube events to metastable superheavy dark matter produced through primordial black hole evaporation, finding viability for

$90\%$5

with a benchmark $90\%$6 PeV and $90\%$7 (Singh et al., 30 Oct 2025). These models do not require a local counterpart and instead interpret the event as part of a diffuse cosmological neutrino spectrum from late decays (Singh et al., 30 Oct 2025).

Not all dark-matter interpretations remain favorable. A detector-level analysis of heavy dark matter decay as the origin of the event finds preferred masses larger than about $90\%$8 PeV and best-fit lifetimes in the range $90\%$9–$2.2^\circ$0 s, but concludes that these regions are in tension with existing gamma-ray and neutrino bounds (Collaboration et al., 8 Jun 2026). It identifies best-fit points such as $2.2^\circ$1 for $2.2^\circ$2, $2.2^\circ$3 for $2.2^\circ$4, and $2.2^\circ$5 for $2.2^\circ$6, but states that the preferred regions in all three channels are excluded by global gamma-ray lower bounds on the lifetime at $2.2^\circ$7 CL (Collaboration et al., 8 Jun 2026).

The event has additionally motivated proposals involving sterile-neutrino reconversion over detector-specific path lengths, speculative electroweak-vacuum-turbulence scenarios in merging black-hole binaries, and other geometry-dependent or transient new-physics mechanisms (Olinto et al., 3 Jul 2025). A PBR study is explicit that if KM3-230213A was “not beginner’s luck,” detector-dependent beyond-standard-model scenarios become compelling targets for future Earth-skimming neutrino follow-up (Olinto et al., 3 Jul 2025).

6. Multimessenger follow-up, caveats, and continuing significance

The observational response to KM3-230213A has been overtly multimessenger. Fermi-LAT searches found no convincing cascade counterpart in GeV gamma rays (Crnogorčević et al., 20 Mar 2025). VERITAS found no significant TeV emission during its 2025 follow-up campaign (Mooney, 29 Sep 2025). HAWC is cited in follow-up work as having reported no significant emission in multiple windows up to one year after the event (Mooney, 29 Sep 2025). Counterpart searches in radio, X-ray, and blazar catalogs have produced plausible associations, but none is secure (Myhr, 8 May 2026).

Several caveats recur across the literature. The strongest interpretations are often model dependent in source distance, source duration, proton composition, or hidden-photon-field assumptions. Many analyses rely on the median event energy $2.2^\circ$8 PeV despite the broad uncertainty interval (Satunin, 13 Feb 2025). Source-specific counterpart studies are limited by the event’s angular uncertainty and by incomplete contemporaneous X-ray or gamma-ray coverage (Oliveira et al., 22 Jul 2025). Population-level fits are intrinsically constrained by single-event statistics (Collaboration et al., 8 Jun 2026). Even the most straightforward source comparisons are conditioned on whether the event is treated as diffuse, transient, point-like, cosmogenic, or exotic (Myhr, 8 May 2026).

Despite these uncertainties, KM3-230213A has already altered the structure of the field. It has sharpened multimessenger tests of hadronic source models, constrained the proton fraction and source evolution of UHECR populations, strengthened neutrino-sector Lorentz-invariance tests, and motivated detector-complementarity studies for future instruments such as IceCube-Gen2, GRAND, POEMMA-Balloon with Radio, and the full KM3NeT/ARCA array (Cermenati et al., 16 Jul 2025). A plausible implication is that the event’s long-term importance will depend less on whether any single current interpretation survives unchanged, and more on whether subsequent UHE neutrino detections reveal a population, a transient class, a cosmogenic component, or a pattern of anomalies requiring new physics.