The ultra-high-energy event KM3-230213A within the global neutrino landscape

TL;DR

This paper reports the detection of an ultra-high-energy neutrino by KM3NeT and analyzes its implications within the global neutrino landscape, comparing with other observatories and exploring possible spectral features.

Contribution

It presents the first observation of an ultra-high-energy neutrino by KM3NeT and performs a joint analysis with IceCube and Pierre Auger data to investigate spectral features and tensions.

Findings

Detection of a neutrino with hundreds of PeV energy by KM3NeT.

Evidence of tension (~2.5-3 sigma) between KM3NeT and other experiments.

Indications of a spectral break in the ultra-high-energy neutrino flux.

Abstract

On February 13th, 2023, the KM3NeT/ARCA telescope detected a neutrino candidate with an estimated energy in the hundreds of PeVs. In this article, the observation of this ultra-high-energy neutrino is discussed in light of null observations above tens of PeV from the IceCube and Pierre Auger observatories. Performing a joint fit of all experiments under the assumption of an isotropic $E^{-2}$ flux, the best-fit single-flavour flux normalisation is $E^2 \Phi^{\rm 1f}_{\nu + \bar \nu} = 7.5 \times 10^{-10}~{\rm GeV cm^{-2} s^{-1} sr^{-1}}$ in the 90% energy range of the KM3NeT event. Furthermore, the ultra-high-energy data are then fit together with the IceCube measurements at lower energies, either with a single power law or with a broken power law, allowing for the presence of a new component in the spectrum. The joint fit including non-observations by other experiments in the ultra-high-energy region shows a slight preference for a break in the PeV regime if the ``High-Energy Starting Events'' sample is included, and no such preference for the other two IceCube samples investigated. A stronger preference for a break appears if only the KM3NeT data is considered in the ultra-high-energy region, though the flux resulting from such a fit would be inconsistent with null observations from IceCube and Pierre Auger. In all cases, the observed tension between KM3NeT and other datasets is of the order of $2.5\sigma-3\sigma$, and increased statistics are required to resolve this apparent tension and better characterise the neutrino landscape at ultra-high energies.