Abstract A significant neutrino event with an estimated energy between 72 PeV and 2.6 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 ultra-high-energy (UHE) proton sources. In this study, we present the best fit to the spectrum and composition of ultra-high-energy cosmic rays (UHECRs) that is consistent with multi-messenger constraints, including the detection of a single neutrino event by the KM3NeT detector at 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 at 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 ∼20% at 20 EeV.
Alhebsi et al. (Thu,) studied this question.
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