Implications of a Cosmogenic Origin of KM3-230213A for Ultra-High-Energy Protons

TL;DR

This paper analyzes a high-energy neutrino event detected by KM3NeT, exploring its implications for the origin of ultra-high-energy cosmic rays, and constrains source models based on multi-messenger data and source evolution.

Contribution

It presents the first combined fit of UHECR spectrum, composition, and neutrino data considering a cosmogenic origin of the event, constraining source evolution and composition models.

Findings

Detection of a single neutrino event suggests strongly evolving UHE proton sources.

Including null observations from other observatories constrains source evolution models.

Proton fraction in UHECRs is approximately 20% at 20 EeV.

Abstract

A significant neutrino event with an estimated energy between $72\,\mathrm{PeV}$ and $2.6\,\mathrm{EeV}$ was recently observed by the KM3NeT experiment (KM3-230213A). When interpreted as cosmogenic in origin, this event can provide constraints on several phenomenological parameters of UHE proton sources. In this study, we present the best fit to the spectrum and composition of UHECRs that is consistent with multi-messenger constraints, including the detection of a single neutrino event by the KM3NeT detector in the energy range of KM3-230213A. From the best fit, we obtain the 68\% CL constraints on the parameters of a two-population model of UHECRs, comprising a mixed-composition population and a subdominant UHE proton population. Our results indicate that the detection of a single neutrino event in the energy range of KM3-230213A solely with the KM3NeT exposure requires strongly evolving UHE proton sources, consistent with high-luminosity active galactic nuclei. On the other hand, including the null observations from the Pierre Auger and IceCube observatories disfavors such strong evolution. In both cases, the observed proton fraction of UHECRs is primarily constrained by the composition data to be $\sim 20\%$ at $20\,\mathrm{EeV}$.