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 altering 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 presents a new opportunity to explore $\nu$SI phenomenology at extreme energies. In this work, we investigate two implications of this observation, assuming the event originates from a diffuse power-law spectrum. First, we find that $\nu$SI-induced spectral distortions can mildly alleviate the tension between the KM3-230213A detection and the previous non-observation of PeV-scale neutrinos in IceCube data. Second, we derive the strongest constraints on the $\tau$-flavored $\nu$SI coupling strength for mediator masses around 100 MeV. Our analysis shows that neutrino telescopes can surpass existing collider bounds in this mass range. In the near future, IceCube-Gen2 is expected to significantly enhance $\nu$SI sensitivity, including regions relevant to alleviating the Hubble and neutrino mass tensions.