Illustration of event KM3-20230213A. This image shows an artist's impression of the KM3-20230213A event, detected on February 13, 2023, and the KM3NeT-ARCA detector as it appeared at the time of observation. The 21 detection lines are each composed of 18 optical modules, represented by the black and yellow balls. Each colored cone indicates that an optical module detected light. The size of the cones is proportional to the intensity of the detected light signal, and the color corresponds to its duration (from purple to blue). The white line symbolizes the neutrino trajectory, and the large blue cone represents the wake of the Cherenkov light. © P. Coyle, CNRS, KM3NeT Collaboration.

In February 2023, the KM3NeT experiment detected a very high-energy event, KM3-230213A, which was identified as a muon of approximately 120 Petaelectronvolts. It was determined that this muon originated from the interaction of a neutrino with an energy of around 220 Petaelectronvolts (or 0.035 Joules!), coming from space. This energy is thirty times higher than that of all neutrinos observed to date. The observation of this neutrino therefore represents our first step into an unexplored sector: that of ultra-high-energy astrophysical neutrinos. These neutrinos are thought to come from interactions of particularly energetic cosmic rays, either with matter or radiation near their sources, or with the cosmic microwave background radiation during their propagation (cosmogenic neutrinos).

This remarkable result was the subject of an article in the journal Nature, as well as its cover story, and was made public on February 12, 2025. It was the subject of presentations by several members of the KM3NeT management, including Miles Lindsey-Clark of the APC (then technical lead of the KM3NeT collaboration), during an international event broadcast on the KM3NeT YouTube channel. The French contributions were then detailed as part of a national event taking place at the CNRS headquarters. Here again, the APC was represented by Antoine Kouchner (chair of the Institute Board of KM3NeT) and Véronique van Elewyck, teacher-researcher at UPCité.

The detection and characterization of this event relied on decades of development of marine neutrino detectors, as well as extensive calibration and analysis work. The APC team has been a major player in the ANTARES experiment, the precursor to KM3NeT, for about 20 years. Since the beginning of KM3NeT's construction, the group's engineers have played a key role in coordinating the development, integration, testing, and deployment of the detectors' instrumentation. In addition to this coordination, the APC team is conducting calibration projects. It has developed a unit (calibration base) equipped, among other things, with a laser for in situ detector calibration and the simulation of one-off events. This unit was deployed at the end of 2024 and was therefore not used for the detection of KM3-230213A, but it will provide essential information for future KM3NeT studies. The APC team also developed test benches to accurately study the detector's absolute response in air and water. Team members also participated in measuring the direction of the KM3-230213A event relative to the detector, a measurement that was crucial in determining its origin.

When KM3-230213A was observed, KM3NeT was approximately 10% of its final size. Construction of the experiment’s two detectors continues, with completion scheduled before 2030. This completion will depend not only on the duration of integration and deployment work but also on securing funding, with current budgets covering 50% of the detector’s cost. KM3NeT is already beginning to collect data and will play a leading role in measuring neutrino properties, particularly mass ordering, as well as in observing astrophysical neutrinos. KM3NeT can already measure the directions of astrophysical neutrinos with unprecedented resolution, and is therefore ideally positioned to identify their sources and, in doing so, determine the origin of cosmic rays.

Team KM3NeT-APC:
Julien Aublin, Bruny Baret, Yvonne Becherini, Théophile Cartraud, Cédric Champion, Joao Coelho, Alexis Coleiro, Stéphane Colonges, Antonio Condorelli, Alexandre Creusot, Corinne Donzaud, Pierre-Alexandre Duverne, Maximilian Eff, Sonia El Hedri, Isabel Goos, Pranjupriya Goswami, Alin Ilioni, Antoine Kouchner, Jean Lesrel, Miles Lindsay-Clark, Enzo Oukacha, Lydie Pavili-Baladine, Santiago Peña Martinez, Ankur Sharma, Benjamin Trocmé, Véronique Van Elewyck, Cédric Verilhac, Sarodia Vydelingum, Isabelle Yvon.

Contact: elhedriAPC.IN2P3.FR (Sonia El Hedri) (APC KM3NeT group leader)