A brief encounter with an energetic cosmic neutrino

Abstract

On 13 February 2023, just after 01:00 UTC, a high-energy muon passed through the sensor field of the seawater-based KM3NeT neutrino experiment, stimulating bursts of Cherenkov light that triggered 28,086 detections. The event was designated KM3-230213A (Nature 638, 376–382; 2025). The path of the muon is shown in red in the image, with the likely shape of the Cherenkov light cones in blue. Muons lose energy through stochastic radiative processes such as bremsstrahlung, pair production and photonuclear reactions; these processes spark electromagnetic cascades that generate Cherenkov light, which is picked up by optical detector modules that are strung at intervals along 21 detection lines (shown as vertical lines). Each optical module contains 31 photomultiplier tubes that enable 4π steradian coverage. In the image, the detected events are represented by spheres clustered around the optical modules and aligned along the orientation of the photomultipliers, with the colours indicating the time since the first triggered hit (a close-up is seen in panel b). From the sequence of hits, the direction and energy of the muon could be determined.

Extrapolating the path of the neutrino into space resulted in no evident transient object that could have been a potential origin. Active galactic nuclei and blazars are the likely sources of cosmic neutrinos, and despite twelve being found within the 99% confidence region of KM3-230213A, none was considered compelling. Another possible source is the interaction of ultra-high-energy cosmic rays with extragalactic background light or the cosmic microwave background, creating so-called cosmogenic neutrinos. None have been detected previously.