First ultra-high-energy neutrino detected in deep-sea telescope – Phys.org
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February 12, 2025
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An extraordinary event consistent with a neutrino with an estimated energy of about 220 PeV (220 x 1015 electron volts or 220 million billion 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 and provides the first evidence that neutrinos of such high energies are produced in the universe. After long and meticulous work to analyze and interpret the experimental data, the international scientific collaboration of KM3NeT reports the details of this discovery in an article in Nature.
The detected event was identified as a single muon which crossed the entire detector, inducing signals in more than one third of the active sensors. The inclination of its trajectory combined with its enormous energy provides compelling evidence that the muon originated from a cosmic neutrino interacting in the vicinity of the detector.
“KM3NeT has begun to probe a range of energy and sensitivity where detected neutrinos may originate from extreme astrophysical phenomena. This first ever detection of a neutrino of hundreds of PeV opens a new chapter in neutrino astronomy and a new observational window on the universe,” comments Paschal Coyle, KM3NeT Spokesperson at the time of the detection, and researcher at CNRS Center National de la Recherche Scientifique—Center de Physique des Particules de Marseille, France.
The high-energy universe is the realm of cataclysmic events such as accreting supermassive black holes at the center of galaxies, supernova explosions, gamma-ray bursts, all, as yet, not fully understood. These powerful cosmic accelerators generate streams of particles called cosmic rays.
Some cosmic rays may interact with matter or photons around the source, to produce neutrinos and photons. During the journey of the most energetic cosmic rays across the universe, some may also interact with photons of the cosmic microwave background radiation, to produce extremely energetic “cosmogenic” neutrinos.
“Neutrinos are one of the most mysterious of elementary particles. They have no electric charge, almost no mass and interact only weakly with matter. They are special cosmic messengers, bringing us 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 at the time of the detection, researcher at the INFN National Institute for Nuclear Physics, Italy.
Although neutrinos are the second most abundant particle in the universe after photons, their weak interaction with matter makes them very hard to detect and requires enormous detectors.
The KM3NeT neutrino telescope, currently under construction, is a giant deep-sea infrastructure distributed across two detectors, ARCA and ORCA. In its final configuration, KM3NeT will occupy a volume of more than one cubic kilometer.
KM3NeT uses sea water as the interaction medium for neutrinos. Its high-tech optical modules detect the Cherenkov light, a bluish glow that is generated during the propagation through the water of the ultra-relativistic particles produced in neutrino interactions.
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“To determine the direction and energy of this neutrino required a precise calibration of the telescope and sophisticated track reconstruction algorithms. Furthermore, this remarkable detection was achieved with only one tenth of the final configuration of the detector, demonstrating the great potential of our experiment for the study of neutrinos and for neutrino astronomy,” comments Aart Heijboer, KM3NeT Physics and Software Manager at the time of the detection, and researcher at Nikhef National Institute for Subatomic Physics, The Netherlands.
The KM3NeT/ARCA (Astroparticle Research with Cosmics in the Abyss) detector is mainly dedicated to the study of the highest energy neutrinos and their sources in the universe.
It is located at 3,450 m depth, about 80 km from the coast of Portopalo di Capo Passero, Sicily. Its 700 m high detection units (DUs) are anchored to the seabed and positioned about 100 m apart.
Every DU is equipped with 18 Digital Optical Modules (DOM) each containing 31 photomultipliers (PMT
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