The KM3NeT event: a primordial high energy neutrino?

TL;DR

This paper explores the hypothesis that the high energy neutrino detected by KM3NeT originated from primordial relics decaying or annihilating in the early Universe, leading to distinctive spectral features.

Contribution

It introduces a novel scenario where primordial relics produce high energy neutrinos with a sharp spectral feature, reducing tension with other experimental null results.

Findings

The scenario can explain the KM3NeT event with a small flux at other energies.

Interactions with background neutrinos and final state radiation modify the neutrino spectrum.

The scenario leaves potential imprints in the CMB without predicting excess gamma-ray flux.

Abstract

High energy neutrinos can be injected in the early Universe from the decay or annihilation of long lived primordial relics. We analyse the possibility that the ultrahigh energy neutrino event recently observed by the KM3NeT neutrino telescope could have such an origin. This possibility has the advantage of leading to a sharp spectral feature in a way that the neutrino flux can be small at all energies except at the KM3NeT event energy. Thus, along this scenario the tension with null results from other experiments is reduced with respect to the usual power law case analysed by the KM3NeT and IceCube experiments. At such energies and for an emission around the recombination time, interactions of these neutrinos with background neutrinos prove to be relevant and must be determined from the development of a dedicated code. These interactions, as well as final state radiation processes, modify the spectrum. Interestingly, it turns out that the scenario can also leave an imprint in the CMB that could be probed in the near future. Interestingly too, this scenario does not predict an associated $\gamma$-ray flux beyond observation. All in all we do find that the high energy neutrino could be a primordial high energy neutrino, provided it has been produced around the recombination time or later.