Scientists have captured an unprecedented glimpse into the extreme universe with the detection of the highest-energy neutrino ever observed, a particle carrying more than 220 million billion electron volts of energy. This remarkable discovery, announced today in Nature, opens a new window into the most violent phenomena in our cosmos.
The extraordinary particle was detected on February 13, 2023, by the KM3NeT neutrino telescope, a massive detector array nestled deep in the Mediterranean Sea. This remarkable observation provides the first evidence that neutrinos of such enormous energies exist in nature.
“KM3NeT has begun to probe a range of energy and sensitivity where detected neutrinos may originate from extreme astrophysical phenomena,” said Paschal Coyle, KM3NeT Spokesperson at the time of the detection and researcher at CNRS Centre de Physique des Particules de Marseille. “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.”
Neutrinos are among the most enigmatic particles in physics. Despite being the second most abundant particle in the universe after photons, they are notoriously difficult to detect because they rarely interact with matter. This ghostly nature makes them valuable cosmic messengers, carrying information from the farthest reaches of space without being deflected by magnetic fields or absorbed by interstellar matter.
“Neutrinos are one of the most mysterious of elementary particles,” explains Rosa Coniglione, KM3NeT Deputy-Spokesperson at the time of the detection and researcher at the INFN National Institute for Nuclear Physics. “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.”
The detection required an extraordinary instrument. KM3NeT uses the deep Mediterranean Sea as a detection medium, employing thousands of sensitive light detectors to watch for the faint blue glow produced when neutrinos interact with water molecules. When complete, the detector will occupy more than a cubic kilometer of seawater.
What makes this discovery particularly remarkable is that it occurred when only about one-tenth of the final detector was operational. “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 and researcher at Nikhef National Institute for Subatomic Physics.
The particle’s extreme energy suggests two intriguing possibilities about its origin. It might have been produced directly by some of the most powerful cosmic accelerators in the universe, such as supermassive black holes at the centers of galaxies. Alternatively, it could be the first detection of a “cosmogenic” neutrino, created when ultra-high-energy cosmic rays interact with background radiation that fills the universe.
The discovery highlights the remarkable technical achievements required for modern neutrino astronomy. “The scale of KM3NeT, eventually encompassing a volume of about one cubic kilometre with a total of about 200,000 photomultipliers, along with its extreme location in the abyss of the Mediterranean Sea, demonstrates the extraordinary efforts required to advance neutrino astronomy and particle physics,” notes Miles Lindsey Clark, KM3NeT Technical Project Manager and research engineer at the CNRS Astroparticle and Cosmology laboratory.
While the source of this ultra-high-energy neutrino remains unknown, its detection marks a significant milestone in our ability to observe the most extreme events in the cosmos. The ongoing expansion of KM3NeT will improve its sensitivity and enhance its ability to pinpoint cosmic neutrino sources, making it a crucial contributor to multi-messenger astronomy.
This discovery represents a collaboration of more than 360 scientists, engineers, technicians and students from 68 institutions across 21 countries. The project is recognized as a priority research infrastructure for Europe and has received funding through various European research and innovation programmes.
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