Probing the Cosmic-Ray Proton Fraction with Cosmogenic Multi-PeV Neutrinos and Gamma-rays Constraints

The recent detection of a multi-PeV neutrino event by KM3NeT/ARCA opens a new window into the origin of ultra-high-energy cosmic rays (UHECRs). We revise the possibility of a cosmogenic origin of this neutrino by analysing the constraints induced by cascaded photons onto the isotropic gamma-ray background measured by the Fermi-LAT collaboration. We consider two scenarios: the first one, at low energies, with a primary proton population saturating the Pierre Auger Observatory data. The second one, the high-energy scenario, is a sub-population of protons contaminating the UHECR flux. Remarkably, the cascaded photons constrain the LE scenario which cannot explain the KM3-230213A event, if the cosmological evolution of the sources is too strong, because the corresponding gamma-ray spectrum would overshoot the corresponding measurements. By contrast, the high-energy scenario is allowed by current cosmic-rays, neutrinos and gamma-rays data and only future constraints on the UHECR composition will further constrain this scenario.