An extraordinary event consistent with a neutrino with an estimated energy of about 220 PeV (220 x 1015 electron volts) was detected on February 13, 2023, by the ARCA detector of the kilometer cubic neutrino telescope (KM3NeT) in the deep sea.
This event, named KM3-230213A, is the most energetic neutrino ever observed, providing the first evidence that such high-energy neutrinos are produced in the universe. After extensive analysis, the international scientific collaboration of KM3NeT published the details of this discovery in Nature.
The detected event was identified as a single muon that traversed the entire detector, triggering signals in more than a third of the active sensors. The inclination of its trajectory and its enormous energy strongly suggest that the muon originated from a cosmic neutrino interacting near the detector.
KM3NeT has begun to explore a range of energy and sensitivity where detected neutrinos may originate from extreme astrophysical phenomena. This first detection of a neutrino of hundreds of PeV opens a new chapter in neutrino astronomy and offers a new observational window on the universe, according to Paschal Coyle, KM3NeT Spokesperson and researcher at CNRS, Marseille, France.
The high-energy universe is the realm of cataclysmic events such as accreting supermassive black holes, supernova explosions, and gamma-ray bursts, which are not yet fully understood. These powerful cosmic accelerators generate streams of particles called cosmic rays.
Some cosmic rays may interact with matter or photons around their source, producing neutrinos and photons. During their journey across the universe, the most energetic cosmic rays can interact with photons of the cosmic microwave background radiation, producing extremely energetic cosmogenic neutrinos.
Neutrinos are among the most mysterious elementary particles. They have no electric charge, almost no mass, and interact weakly with matter. They are special cosmic messengers, providing unique information on the mechanisms involved in the most energetic phenomena and allowing us to explore the farthest reaches of the universe, explains Rosa Coniglione, KM3NeT Deputy-Spokesperson and researcher at INFN, Italy.
Although neutrinos are the second most abundant particles in the universe after photons, their weak interaction with matter makes them hard to detect, requiring enormous detectors.
The KM3NeT neutrino telescope, currently under construction, is a giant deep-sea infrastructure with two detectors, ARCA and ORCA. In its final form, KM3NeT will occupy a volume of more than one cubic kilometer.
KM3NeT uses seawater as the interaction medium for neutrinos. Its high-tech optical modules detect the Cherenkov light, a bluish glow generated during the propagation of ultra-relativistic particles produced in neutrino interactions.
Determining the direction and energy of this neutrino required precise telescope calibration and sophisticated track reconstruction algorithms. This remarkable detection was achieved with only a tenth of the final detector configuration, demonstrating the experiment's potential for neutrino studies, as noted by Aart Heijboer, KM3NeT Physics and Software Manager.
The KM3NeT/ARCA (Astroparticle Research with Cosmics in the Abyss) detector focuses on studying the highest energy neutrinos and their sources. Located at a depth of 3,450 m, about 80 km from Sicily's coast, its detection units are equipped with Digital Optical Modules, each containing photomultipliers. In its final configuration, ARCA will have 230 DUs with data transmitted via a submarine cable to the INFN Laboratori Nazionali del Sud.
The KM3NeT/ORCA (Oscillation Research with Cosmics in the Abyss) detector is optimized to study fundamental neutrino properties. Located at 2,450 m depth, about 40 km from the coast of Toulon, France, it will comprise 115 DUs. Data collected by ORCA are sent to the shore station at La Seyne Sur Mer.
The KM3NeT's scale, eventually covering about one cubic kilometer with around 200,000 photomultipliers, along with its location in the Mediterranean Sea's abyss, showcases the extraordinary efforts required to advance neutrino astronomy and particle physics, comments Miles Lindsey Clark, KM3NeT Technical Project Manager.
The detection of this event results from extensive collaboration between many international teams of engineers, technicians, and scientists.
This ultra-high energy neutrino may originate directly from a powerful cosmic accelerator, or it could be the first detection of a cosmogenic neutrino. However, based on this single neutrino, its origin remains unclear.
Future observations will focus on detecting more such events to build a clearer picture. The ongoing expansion of KM3NeT with additional detection units and data acquisition will enhance its sensitivity and ability to pinpoint cosmic neutrino sources, making it a leading contributor to multi-messenger astronomy.
The KM3NeT Collaboration involves over 360 scientists, engineers, technicians, and students from 68 institutions across 21 countries worldwide.
