Cosmic neutrinos: the KM3NeT record and the search for its source

What are cosmic neutrinos?

A neutrino is an electrically neutral elementary particle that very rarely interacts with matter. Magnetic fields do not bend its path. A neutrino can therefore carry information out of a dense region where light is absorbed or repeatedly scattered. For astronomers, this offers another independent signal with which to investigate natural particle accelerators.

Neutrinos also arise in nuclear reactions: the Sun continuously produces them as hydrogen is converted into helium. The record KM3NeT event, however, belongs to a very different energy range. Here, researchers look for evidence of extremely energetic protons and atomic nuclei colliding with gas or photons. These collisions create unstable particles whose decays produce neutrinos.

What the detector saw beneath the Mediterranean

On 13 February 2023, the ARCA detector off Sicily recorded an unusually bright track. At the time, 21 vertical lines of optical modules were operating. The sensors recorded more than 28,000 light hits in a short interval. Following extensive checks, the collaboration announced the discovery on 12 February 2025 and published its analysis in Nature. The designation KM3-230213A contains the event date.

The sensors did not photograph the neutrino itself. According to the reconstruction, it interacted with matter near the detector and produced a muon, a charged particle and heavier relative of the electron. The muon crossed the detector and left Cherenkov light: it travelled faster than light propagates through water. This does not violate the speed of light in a vacuum.

The muon track was nearly horizontal. This geometry matters when checking the signal's origin: a thick layer of water lies above the detector, and the shallow path crosses even more water, suppressing atmospheric muons. Conversely, Earth is no longer fully transparent to neutrinos at such high energies. The horizon is therefore a particularly useful place to look for these events, rather than through the entire planet.

What an energy of 220 PeV means

From the amount and distribution of light, the team estimated the muon's energy at about 120 petaelectronvolts. The parent neutrino must have had still more energy. To connect the observed track to the original particle, the researchers simulated interactions and the detector response. For their assumed incoming neutrino spectrum, the median estimate was 220 PeV; the central 68% of simulated energies lay between 110 and 790 PeV.

One petaelectronvolt equals 10¹⁵ electronvolts. The large number describes the energy of a single particle, rather than the power of an explosion near Earth. The spread of estimates shows why 220 PeV should not be read as the output of a precise energy meter. The result depends on seawater transparency, sensor sensitivity and assumptions about the neutrino flux. With these limitations, the event still holds the record for estimated energy among detected neutrinos.

Where the particle might have come from

An origin beyond the Milky Way is considered most likely. Candidates include active galactic nuclei and rare transient sources. Blazars are of particular interest: in these active nuclei, one of the jets points roughly towards Earth. The jet is powered by a system centred on a supermassive black hole. Collisions of accelerated particles in this environment could produce high-energy neutrinos.

There is also a cosmogenic possibility: the neutrino was produced en route when a cosmic ray collided with a background photon in the Universe. Its arrival direction would then not necessarily point to a bright flare at the time of detection. Searches for candidate blazars and other objects have not yet produced an unambiguous identification. Assessing a class of sources and establishing a connection to one particular galaxy are different tasks.

What IceCube checked in 2026

IceCube uses the same principle of detecting secondary particles, but its optical sensors are embedded in Antarctic ice. In a study announced by the team on 23 September 2026, researchers examined 15 years of observations towards KM3-230213A. They searched for steady emission, flares at arbitrary times and signals in specified windows around the record event. They found no convincing excess of neutrinos.

This result constrains source models. Some predict that a rare ultra-high-energy event should be accompanied by more lower-energy neutrinos that IceCube could detect. The absence of such a signal does not by itself invalidate the ARCA measurement. The instruments observe the sky differently, and the inference depends on the assumed spectrum, duration of activity and statistics of rare events. As of 5 November, the IceCube study is available as a preprint submitted to Physical Review Letters.

Why the new OMEN project needs the Sun and Moon

On 12 October 2026, the University of Amsterdam announced funding for the OMEN project. Its researchers aim to tie the underwater detector more accurately to sky coordinates. In the original analysis, the directional uncertainty was dominated by knowledge of the detector's orientation. Reducing this systematic uncertainty could narrow the field of potential objects for follow-up observations.

The proposed calibration uses the shadows of the Sun and Moon in the cosmic-ray flux. These bodies block some of the charged primary particles that normally produce atmospheric muons. The resulting deficit of events helps check where the detector is actually pointing. The project combines data analysis with calculations of particle trajectories. The funding announcement does not mean that a new localisation has already been obtained.

Does the event establish new physics?

The unusual energy encourages theorists to consider more exotic scenarios, including the decay of superheavy particles. Being able to fit a model to one event, however, does not establish the existence of new particles or extra dimensions. Tests require other observable consequences: the neutrino spectrum, the distribution of arrival directions, accompanying gamma rays and the rate of further events.

In Carla Distefano's review, posted on arXiv on 29 October 2026, an unambiguous electromagnetic counterpart remains unidentified. This is a review of existing results, rather than a report of a new record particle. The main next step is to collect more events and refine the arrival direction. Joint observations of light, neutrinos and, where relevant, gravitational waves allow one physical picture to be tested in several independent ways.