Strong Neutrino-Nucleon Interactions at Ultrahigh Energies as a Solution to the GZK Puzzle

After a short review of the ultrahigh energy cosmic ray puzzle - the apparent observation of cosmic rays originating from cosmological distances with energies above the expected Greisen-Zatsepin-Kuzmin cutoff 4x10^{19} eV - we consider strongly interacting neutrino scenarios as an especially interesting solution. We show that all features of the ultrahigh energy cosmic ray spectrum from 10^{17} eV to 10^{21} eV can be described to originate from a simple power-like injection spectrum of protons, under the assumption that the neutrino-nucleon cross-section is significantly enhanced at center of mass energies above ~ 100 TeV. In such a scenario, the cosmogenic neutrinos produced during the propagation of protons through the cosmic microwave background initiate air showers in the atmosphere, just as the protons. The total air shower spectrum induced by protons and neutrinos shows excellent agreement with the observations. We shortly discuss TeV-scale extensions of the Standard Model which may lead to a realization of a strongly interacting neutrino scenario. We emphasize, however, that such a scenario may even be realized within the standard electroweak model: electroweak instanton/sphaleron induced processes may get strong at ultrahigh energies. Possible tests of strongly interacting neutrino scenarios range from observations at cosmic ray facilities and neutrino telescopes to searches at lepton nucleon scattering experiments.