KM3NeT’s two neutrino detectors?— one off the coast of Sicily, the other near southern France — are still under construction but already collecting data. Both contain cables hundreds of meters tall, which are strung with bundles of light sensors anchored to the seafloor.

When cosmic neutrinos interact with matter in or near a KM3NeT detector, they spawn charged particles such as muons. As those muons careen through water, they give off feeble flashes of bluish light that KM3NeT’s sensors can pick up. Clocking when different sensors spot this light can reveal a particle’s path; the brightness of the blue hue reflects the particle’s energy.

On February 13, 2023, the detector near Sicily was run through by an extremely energetic muon traveling nearly parallel to the horizon. At the time, only 21 of the planned 230 sensor cables were in place. Based on the muon’s energy and trajectory, KM3NeT scientists determined it must have been spawned by a neutrino from space rather than a particle from the atmosphere.

Simulations suggest the neutrino’s energy was around 220 petaelectron volts. The previous record holder boasted around 10 petaelectron volts.

The KM3NeT researchers estimate that they expect to see a neutrino of this caliber once every 70 years or so.

【1】"A cosmic neutrino of unknown origins smashes energy records"

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