Origin of the $220\,\mathrm{PeV}$ neutrino event KM3-230213A
In plain words
In 2023 a detector in the Mediterranean recorded a neutrino about 30 times more energetic than any seen before, while the larger IceCube detector has seen none like it.
Precise statement
KM3-230213A (reconstructed neutrino energy $\sim 220\,\mathrm{PeV}$, 72 PeV to 2.6 EeV at 90 percent confidence) implies, if steady and isotropic, a flux in tension at about 2.5 to 3 $\sigma$ with IceCube non-detections above 10 PeV. Determine whether it is a cosmogenic neutrino, emission from a transient point source such as a flaring blazar in the error region, or an upward fluctuation, and which flux is consistent with both detectors. Answer: a joint flux estimate and a source attribution with probabilities.
What would settle it
Joint IceCube-KM3NeT analysis with added exposure, and detection or exclusion of further events above $1e17\,\mathrm{eV}$.
Status in the literature
Unverified note
2025: KM3NeT published the event; follow-up analyses found that a steady cosmogenic origin requires a flux in tension with IceCube limits, leaving transient and fluctuation explanations.