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

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 $20\,\mathrm{EeV}$.

Figures (3)

• Figure 1: The CR spectrum (top) and composition observables (bottom) of the KM3NeT-only (left) and joint (right) fits shown alongside data by Auger auger_spec2021auger_fd_icrc23. The contribution of different mass groups is shown with $[A_{\text{min}}, A_{\text{max}}]$, and colored bands indicate the 68% CL range. The Galactic component is added to our model from thoudam2016cosmic, albeit with a free normalization. In the lower panels, the epos-lhc hadronic interaction model is used to interpret different nuclei (grey dashed lines) and the model-predicted composition (solid blue line) into $\langle X_{\text{max}}\rangle$ and $\sigma(X_{\text{max}})$.
• Figure 2: Cosmogenic neutrinos (solid lines) and diffuse electromagnetic-cascade gamma rays (dashed lines) of the KM3NeT-only (left) and joint (right) fits. All neutrino fluxes are per-flavor, assuming flavor equipartition post-propagation. Contributions from the mixed and pure-proton populations are shown in purple and orange, respectively, with bands indicating the 68% CL range. We show the IGRB measured by Fermi-LAT, IceCube's high-energy event samples (High-Energy Starting Events (HESE), Northern-Sky Tracks (NST), and the Glashow resonance event), as well as the 90% CL neutrino upper limits reported by IceCube and Auger. We also show the $E^{-2}$ flux associated with KM3-230213A (see main text), for which the horizontal error bar represents the 90% CL interval on the estimated neutrino energy, and the vertical error bars represent the 68%, 95%, and 97.5% Feldman-Cousins confidence intervals feldman1998unified.
• Figure 3: The 68% CL range in the model-predicted integral flux of UHE gamma rays and current 95% CL upper limits posed by Auger auger_UHE_gamma_ULauger_UHE_gamma_UL_2024 and the Telescope Array TA_UHE_gamma_UL.