Observation of an Ultra-high Energy Cosmic Neutrino with KM3NeT

High-energy cosmic neutrinos provide unique insights into astrophysical phenomena, as they are not deflected by magnetic fields and rarely absorbed by interstellar matter. These detections are also relevant for particle physics, probing neutrino interactions at energies far beyond terrestrial accelerators. Here we report the observation of an exceptionally high-energy event by KM3NeT, the deep-sea neutrino telescope in the Mediterranean Sea. A muon was detected with an estimated energy of 120 PeV. Given its enormous energy and near-horizontal direction, the muon most probably originated from the interaction of a neutrino of even higher energy in the vicinity of the detector. The cosmic neutrino energy spectrum measured up to now falls steeply with energy. However, this event's energy grealy exceeds that of any previously detected neutrino. This suggests that this neutrino may have originated in a different cosmic accelerator than the lower-energy neutrinos, or this may be the first detection of a cosmogenic neutrino, resulting from the interactions of ultra-high-energy cosmic rays with background photons in the Universe.