Neutrino transition dipole moments in light of the KM3NeT event

In 2023, KM3NeT observed the highest-energy neutrino seen to date. The most probable estimate places the energy of the neutrino at an extreme value of 220 PeV, and energies up to 2.6 EeV are contained in the 90% probability interval. Even though the source of the signal cannot be ascertained with the available data, this observation already opens unique possibilities to test new physics. Here, we study the implications for the transition magnetic dipole portal connecting active and sterile neutrinos. We remain agnostic regarding the origin of the neutrino and focus on local effects. Concretely, we study the impact of efficient active-sterile transitions on the energy degradation experienced by the neutrino along its path to the detector at Earth. We find that the ultrahigh-energy neutrino event does not add relevant information for sterile neutrinos with masses less than a TeV. However, the observation becomes important at higher masses and can provide the leading constraint in that regime.