Scientists with the KM3NeT collaboration reported evidence of the most energetic neutrino detected to date, based on a flash-filled particle track recorded deep beneath the Mediterranean Sea.
The event occurred on February 13, 2023, when a muon crossed KM3NeT’s ARCA detector off Sicily. The array’s optical sensors registered 28,086 hits across 21 operating detector lines. Researchers estimated the muon’s energy at 120 petaelectronvolts, with a wide uncertainty range, and concluded that it most likely came from an even more energetic neutrino interacting near the detector.
The Associated Press reported that the inferred neutrino carried roughly 30 times the energy of the previous record holder. Its precise birthplace remained unknown, although scientists considered an origin beyond the Milky Way likely. The results were published in Nature on February 12, nearly two years after the observation.
Neutrinos have almost no mass and interact only weakly with matter, allowing most to cross enormous distances without being absorbed or deflected. That makes their direction useful to astronomers seeking the environments that accelerate cosmic rays to extreme energies. It also makes the particles difficult to detect directly. Underwater observatories instead measure light produced by charged particles, such as muons, created when a neutrino occasionally strikes matter nearby.
KM3NeT’s paper said the event could have come from a type of cosmic accelerator different from those responsible for lower-energy neutrinos. Another possibility is that it was cosmogenic: produced when ultra-high-energy cosmic rays encountered background photons during their journey through space. One detection could not distinguish between those explanations.
ARCA was still under construction when the event was captured and sits about 3,450 metres below the sea surface. Detecting such an extreme particle with only part of the array operating suggested that the completed observatory could reveal more about the highest-energy processes in the universe. Independent physicists cautioned, however, that additional observations would be needed before researchers could identify a source or establish a broader population.
