Deep-Sea KM3NeT Laboratory Detects Record 220 PeV Neutrino

Scientists at the kilometre-cubic neutrino telescope have observed the highest-energy neutrino so far, deep beneath Mediterranean waters, heralding astrophysical insights.

• ARCA detector in the Mediterranean Sea observes 220 PeV cosmic neutrino
• KM3NeT laboratory uses advanced optical modules to track elusive particles
• International collaboration signals new era in neutrino astronomy opportunities for multi-messenger astronomy and international research collaboration

The detection of an estimated 220 PeV (220 x 10^15 electron volts) neutrino by the ARCA detector marks a pivotal moment for the KM3NeT neutrino telescope, a deep-sea research facility with an eventual volume of more than one cubic kilometre. Researchers say the observation, named KM3-230213A, is “the most energetic neutrino ever observed.” Its detection provides compelling evidence that neutrinos at such high energies exist in the Universe, hinting at their potential origin in extremely violent astrophysical environments.

Neutrinos are uniquely challenging to study. Having almost no mass and lacking electric charge, they can pass through ordinary matter with minimal interaction, requiring detectors on an immense scale. KM3NeT is one such instrument, anchored in the abyss of the Mediterranean Sea, where deep water acts as the interaction medium. The data collected are channelled to shore-based laboratories via submarine cables, ensuring that every flash of Cherenkov light produced by passing cosmic particles is scrutinised. This new discovery reaffirms the rationale for constructing and expanding the KM3NeT facility, as scientists work to understand cosmic neutrino sources and their role in astrophysical processes.

Constructing a Subsea Observatory

KM3NeT comprises two main detectors: ARCA (Astroparticle Research with Cosmics in the Abyss) and ORCA (Oscillation Research with Cosmics in the Abyss). ARCA, located about 80 km off the coast of Portopalo di Capo Passero, Sicily, sits at a depth of around 3450 m. Its detection units rise 700 m above the seabed and are spaced about 100 m apart, each equipped with 18 digital optical modules fitted with arrays of photomultipliers. Once completed, ARCA alone will consist of 230 such units.

The kilometres of cabling and precise deployments illustrate the technical feats necessary to build a detector that can withstand ocean currents and high-pressure conditions. As KM3NeT Technical Project Manager at the time of detection, Miles Lindsey Clark, notes: “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. The detection of this event is the result of a tremendous collaborative effort between many international teams of engineers, technicians and scientists.”

ORCA, meanwhile, is situated at a depth of 2450 m near Toulon, France, and focuses on studying the fundamental properties of neutrinos themselves. Together, ARCA and ORCA establish a cross-border initiative that exemplifies large-scale scientific collaboration, uniting more than 360 scientists, engineers, technicians and students from 21 countries. The ultimate goal is to cover vast volumes of ocean and capture fleeting neutrino signals from all corners of the Universe.

Discoveries at the Frontier

KM3-230213A was identified as a single muon crossing the ARCA detector, triggering signals across more than one third of the active sensors. The sheer energy involved, combined with the particle’s angle of incidence, offers strong evidence that it originated in a cosmic neutrino interaction close to 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,” says KM3NeT Spokesperson at the time of detection, Paschal Coyle.

Accurate directional and energy measurements of neutrinos necessitate intricate calibration procedures and sophisticated algorithms. “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,” explains Aart Heijboer, KM3NeT Physics and Software Manager at the time of detection.

Investigating extreme cosmic processes is a key driver for neutrino astronomy, whether neutrinos arise from cataclysmic events such as gamma ray bursts or from interactions involving the cosmic microwave background. As Rosa Coniglione, KM3NeT Deputy-Spokesperson at the time of detection, notes: “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.”

The newly observed event paves the way for further detections that will help pinpoint the origins of ultra-high energy neutrinos. With each additional detection unit deployed and more data acquired, KM3NeT promises to remain at the forefront of multi-messenger astronomy, enriching our view of cosmic events through a deep-sea lens.