Clash of the Titans: ultra-high energy KM3NeT event versus IceCube data

KM3NeT has reported a remarkably high-energy through-going muon that lit up about a third of the detector and likely originated from a neutrino with energy exceeding 10 PeV. The key open question is where this event came from and what its source is, especially because IceCube has operated with a much larger effective area for considerably longer yet has not reported neutrinos above 10 PeV.

To address this, the authors quantify the tension between the KM3NeT event and IceCube's absence of similar high-energy events. Their detailed analysis determines the most likely neutrino energy to be in the range 23-2400 PeV. Assuming the neutrino comes from the diffuse isotropic neutrino flux, they find a 3.5σ tension between the two experiments. Alternatively, assuming a cosmogenic origin and considering three representative models, the tension still falls within 3.1-3.6σ.

Among the scenarios considered, the least disfavored is a steady or transient point source, though these still lead to 2.9σ and 2.0σ tensions, respectively. The lack of observed high-energy events in IceCube seriously challenges explaining the KM3NeT event as coming from any known diffuse fluxes, and the results indicate the KM3NeT event is likely the first observation of a new astrophysical source.