The Highest Energy Neutrino Ever Observed

Symbolic picture for the article. The link opens the image in a large view. — Visual impression of the ultra-high energy neutrino event observed in KM3NeT/ARCA. The colours indicate the light seen by the “eyes” on each sensor module, where the different colours represent different observing times. The almost horizontally reconstructed track of the particle is shown as a line from left to right. Credit: The KM3NeT Collaboration

February 12, 2025

Detection with the Deep-Sea Telescope KM3NeT from the abyss of the Mediterranean Sea

From the abyss of the Mediterranean Sea, scientists including astroparticle physicists from the ECAP at the FAU Erlangen-Nürnberg, Germany, used the KM3NeT neutrino telescope to detect a cosmic neutrino with a record-breaking energy of about 220 PeV (220 · 10¹⁵ eV). The ARCA detector, part of the KM3NeT deep-sea observatory, captured this extraordinary event, named KM3-230213A, which is the most energetic neutrino ever observed.
**This discovery provides the first evidence that neutrinos of such extreme energies are produced in the universe,a breakthrough now published in today’s issue of the journal Nature.**

Visual impression of the ultra-high energy neutrino event observed in KM3NeT/ARCA. The colours indicate the light seen by the “eyes” on each sensor module, where the different colours represent different observing times. The almost horizontally reconstructed track of the particle is shown as a line from left to right.
Credit: The KM3NeT Collaboration

Visual impression of the ultra-high energy neutrino event observed in KM3NeT/ARCA. The colours indicate the light seen by the “eyes” on each module, where the different colours represent different observing times. Full Resolution at

https://ecap.nat.fau.de/wp-content/uploads/2025/02/Style1_KM3-230213A_Event_display_with_animation_of_signals_©KM3NeT.mp4

Credit: KM3NeT Collaboration

After meticulous analysis and interpretation of the experimental data, the international scientific collaboration working with the kilometre-cubic KM3NeT neutrino telescope— including scientists from the Erlagnen Centre for Astroparticle Physics (ECAP) at the Friedrich-Alexander-Universität Erlangen-Nürnberg —has identified a signalon 13 February 2023 with an energy 16,000 times greater than the most powerful particle collisions that can be produced by CERN’s Large Hadron Collider.

The detector picked up a single muon that crossed the entire detector, triggering signals in more than a third of the active sensors. The direction in which it travelled, and its strength suggest that the muon came from a cosmicneutrino close to the detector. Neutrinos are the second most abundant particles in the Universe after photons (light). Because they interact very weakly with matter, detecting them requires huge instruments.

The KM3NeT telescope, which is currently being built, is a huge deep-sea structure made up of two parts, ARCA and ORCA. When it is finished, KM3NeT will be more than one cubic kilometre in size. KM3NeT relies on seawater as both a detection medium and a shield against background particles. Its high-tech optical modules detect the Cherenkov light, a bluish glow that is generated during the movement of the extremely fast particles produced in interactions of neutrinos through the water.

The high-energy universe is full of dramatic scenes and events—supermassive black holes, supernova explosions, andgamma-ray bursts—whose inner workings remain largely mysterious. These powerful cosmic accelerators produce streams of cosmic rays, some of which interact with surrounding matter or photons to generate neutrinos and high-energy photons. Other cosmic rays may collide with photons from the cosmic microwave background, creating extremely energetic cosmogenic neutrinos.

KM3NeT is now detecting neutrinos from extreme astrophysical events, exploring previously uncharted energy ranges. “This first detection of a neutrino in the hundreds of PeV range opens a new chapter in neutrino astronomy,” says Paschal Coyle, KM3NeT spokesperson at the time of the detection and a researcher at IN2P3/CNRS in France.One petaelectronvolt (PeV) corresponds to 10¹⁵ or one quadrillion electronvolts. Neutrinos are among the most mysterious elementary particles – they have no electric charge, almost no mass, and interact only weakly with matter. “They are special cosmic messengers that reveal the secrets of the most energetic phenomena in the universe,” adds Rosa Coniglione, deputy spokesperson for KM3NeT at the time of the discovery and a researcher at INFN in Italy.

Determining the direction and energy of this neutrino required precise calibration of the telescope and advanced track reconstruction algorithms. “This remarkable detection was achieved with only a tenth of the final detector configuration, demonstrating the great potential of our experiment,” comments Aart Heijboer, KM3NeT physics and software manager at the time of the detection and a researcher at the National Institute for Subatomic Physics (Nikhef) in the Netherlands.

This high-energy neutrino could have come directly from a powerful cosmic accelerator, or it could be the firstdetection of a cosmogenic neutrino. However, with only one event, its origin remains uncertain. To learn more, astronomers and physicists need to detect more such events. The KM3NeT project is expanding with additional detection units and more data, improving its ability to capture cosmic neutrinos and cementing its role as a key tool in multimessenger astronomy. To put the energies involved in a macroscopic perspective, KM3- 230213A’s energy is about 30 times that required to press a key on a computer keyboard.

“We have known for about a hundred years that particles are accelerated to extremely high energies in our universe, but the sources of these particles and the acceleration mechanisms within them are still largely unknown,” explains Prof Dr Claudio Kopper, head of the KM3NeT group at ECAP and holder of the FAU Chair of Experimental Astroparticle Physics. “Neutrinos allow us to find the sources of these particles. We are also learning to understand the processes that drive such cosmic particle accelerators, as neutrinos are almost not absorbed and can therefore provide very ‘direct’ information from inside such astrophysical accelerators.”

“The KM3NeT project is an enormous undertaking, covering about one cubic kilometre and using around 200,000 photomultipliers,” says Miles Lindsey Clark, KM3NeT Technical Project Manager at the time of detection and research engineer at IN2P3/CNRS in France. “This achievementreflects the tremendous collaborative effort of engineers, technicians, and scientists working in one of the most challenging environments for neutrino research.“

The detection of KM3-230213A marks a major step forward in multimessenger astrophysics, a field that combines observations across the electromagnetic spectrum and beyond – including cosmic rays and neutrinos – to study the most energetic events in the universe.

FAU astroparticle physicists played a leading role in the project from the very beginning. In the case of the event that has now been measured, they contributed in particular to the visualisation and the provision of further data to the scientific community.

An assembly of optical modules which later become a part of the KM3NeT neutrino telescope in the Mediterranean Sea.

Credit: KM3NeT Collaboration

“It is great that we are already making such an observation in the start-up phase, which shows us that we are on the right track with KM3NeT,” says Prof Uli Katz from the Chair of Astroparticle Physics at ECAP, one of the founding fathers of the KM3NeT project.

Credit: KM3NeT Collaboration

Additional Information

The KM3NeT Collaboration brings together more than 360 scientists, engineers, technicians, and students of almost 70 institutions from more than 20 countries all over the world.

KM3NeT has two detectors: ARCA near Sicily and ORCA near Toulon in France. The ARCA detector has 230 units, while the ORCA detector has 115. ARCA units are 700 m high and 100 m apart, while ORCA units are 200 m high with 20 m separations. Each unit contains 18 optical modules with 31 photomultipliers. The data are sent to the shore stations at the INFN Laboratori Nazionali del Sud in Portopalo di Capo Passero and the Laboratoire Sous-marin Provence Méditerranée in La Seyne-sur-Mer via submarine cable.

The following institutions in Germany were involved in the published study:
Friedrich-Alexander-Universität Erlangen-Nürnberg with scientists M. Chadolias, Y. Darras, A. Domi, T. Eberl, T. Gal, N. Geißelbrecht, R. Gracia, K. Graf, C. Haack, L. Hennig, O. Kalekin, U.F. Katz, C. Kopper, R. Lahmann, J. Schnabel, J. Schumann, B. Setter, H. Warnofer, and S. Weissbrod;
Max-Planck-Institut für Radioastronomie with Y.Y. Kovalev, A. Plavin, and E. Ros; and
Julius-Maximilian-Universität Würzburg with S.Buson (also Deutsches Elektronen- Synchrotron DESY), M. Lincetto, and L. Pfeiffer.