In an unprecedented milestone for astrophysics, the cubic-kilometer neutrino telescope (KM3NeT), located in the depths of the sea, has detected a neutrino with an estimated energy of approximately 220 petaelectronvolts (PeV), equivalent to 220 million billion electronvolts.
This extraordinary event, identified on February 13, 2023, by the KM3NeT ARCA detector, has been named KM3-230213A and has become the most energetic neutrino ever recorded. Its discovery provides the first direct evidence of the existence of such ultra-high-energy neutrinos in the Universe.
The official announcement of this discovery has been published in the journal Nature, following a meticulous and prolonged analysis of experimental data conducted by the international KM3NeT collaboration.
A Messenger from the Extreme Universe
The detected event has been identified as a muon that traversed the entire detector, leaving a signal in more than a third of the active sensors. The inclination of its trajectory, combined with its colossal energy, constitutes compelling evidence that the muon originated from the interaction of a cosmic neutrino in the vicinity of the detector.
Dr. Paschal Coyle, spokesperson for KM3NeT at the time of detection and researcher at the National Center for Scientific Research (CNRS) in France, highlighted the significance of the finding: “KM3NeT has begun exploring an energy and sensitivity range where the detected neutrinos could originate from extreme astrophysical phenomena. The detection of a neutrino with hundreds of PeV opens a new chapter in neutrino astronomy and offers us an unprecedented observational window into the Universe.”
The Origin of High-Energy Neutrinos
The high-energy Universe is the setting for catastrophic events such as the accretion of supermassive black holes at the hearts of galaxies, supernova explosions, and gamma-ray bursts—processes that are not yet fully understood.
These cosmic sources can act as natural particle accelerators, generating cosmic rays that, upon interacting with matter or radiation near their origin, produce neutrinos and photons.
Additionally, the most energetic cosmic rays can collide with cosmic microwave background photons, giving rise to ultra-relativistic neutrinos known as “cosmogenic” neutrinos.
Dr. Rosa Coniglione, deputy spokesperson for KM3NeT at the time of detection and researcher at the National Institute for Nuclear Physics (INFN) in Italy, emphasized the uniqueness of neutrinos in cosmic exploration: “Neutrinos are among the most enigmatic elementary particles. They have no electric charge, possess an almost negligible mass, and interact extremely weakly with matter. They are exceptional cosmic messengers, providing us with unique information about the mechanisms involved in the most energetic phenomena and allowing us to study the most distant regions of the Universe.”
A State-of-the-Art Detector in the Ocean Depths
Despite being the second most abundant particle in the Universe, after photons, neutrinos are extremely challenging to detect due to their weak interaction with matter. Therefore, colossal telescopes are required. KM3NeT, currently under construction, is a massive underwater infrastructure distributed across two detectors: ARCA and ORCA.
Once completed, it will occupy a volume exceeding one cubic kilometer and will utilize seawater as the interaction medium for neutrinos. Its sophisticated optical modules allow the detection of Cherenkov light—a characteristic blue glow generated by ultra-relativistic particles propagating through water.
Aart Heijboer, head of Physics and Software for KM3NeT at the time of discovery and researcher at the National Institute for Subatomic Physics (Nikhef) in the Netherlands, underscored the technological challenge of the finding: “To determine the direction and energy of this neutrino, it was necessary to calibrate the telescope with great precision and develop complex trajectory reconstruction algorithms. The most impressive aspect is that this detection was achieved with only one-tenth of the detector’s final configuration, demonstrating the experiment’s enormous potential for neutrino astronomy.”
The Future of Neutrino Astronomy
The KM3NeT/ARCA detector (Astroparticle Research with Cosmics in the Abyss) is designed to study ultra-high-energy neutrinos and their cosmic sources. Located 3,450 meters deep, about 80 km off the coast of Sicily, it consists of detection units 700 meters tall. Its counterpart, KM3NeT/ORCA (Oscillation Research with Cosmics in the Abyss), is optimized for analyzing fundamental neutrino properties and is situated 2,450 meters deep, about 40 km off the coast of Toulon, France.
Engineer Miles Lindsey Clark, technical project manager at the time of detection and researcher at the CNRS Laboratory for Astroparticles and Cosmology in France, highlighted the magnitude of the collective effort: “The scale of KM3NeT and its extreme location in the depths of the Mediterranean reflect the extraordinary effort required to advance neutrino astronomy and particle physics.”
The discovery of this neutrino could mark the beginning of a new era in cosmic exploration. Future observations will focus on detecting similar events to unravel the origin of these cosmic messengers and expand the horizons of multi-messenger astronomy.
SOURCES
The KM3NeT Collaboration. Observation of an ultra-high-energy cosmic neutrino with KM3NeT. Nature 638, 376–382 (2025). doi.org/10.1038/s41586-024-08543-1
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