Implications of the KM3NeT Ultrahigh-energy Event on Neutrino Self-interactions

Neutrino self-interactions ({\nu}SI) mediated by light bosonic particles can produce characteristic spectral dips in astrophysical neutrino fluxes, thereby modifying the expected energy spectrum. The high-energy astrophysical neutrino spectrum has been extensively used to probe {\nu}SI models through these distinctive features. The recent detection of the ultrahigh-energy event KM3-230213A offers a new opportunity to explore {\nu}SI phenomenology at extreme energies. In this work, we investigate two implications of this observation under the assumption that the event originates from a diffuse power-law spectrum. First, we find that {\nu}SI induces spectral distortions that can mildly alleviate tensions between the KM3-230213A detection and the previous non-observation of PeV-scale neutrinos in IceCube data. Second, we derive new constraints on the {\nu}SI coupling strength for mediator masses around 100 MeV. Our analysis shows that neutrino telescopes can surpass existing collider constraints in this mass range. In the near future, IceCube-Gen2 is expected to substantially enhance the sensitivity to {\nu}SI over a broader range of parameter space.