Strongly Interacting Neutrinos as the Highest Energy Cosmic Rays: A Quantitative Analysis

Scattering processes in the cosmic microwave background limit the propagation of ultra high energy charged particles in our Universe. For extragalactic proton sources resonant photopion production results in the famous Greisen-Zatsepin-Kuzmin (GZK) cutoff at about 4x10^10 GeV expected in the spectrum observed on Earth. The faint flux of ultra high energy cosmic rays of less than one event per year and cubic kilometer and the large systematic uncertainties in the energy calibration of cosmic ray showers is a challenge for cosmic ray observatories and so far the GZK cutoff has not been unambiguously confirmed. We have investigated the possibility that the primaries of super-GZK events are strongly interacting neutrinos which are not subject to the GZK cutoff. For the flux of protons and neutrinos from extragalactic optically thin sources and a flexible parameterization of the neutrino-nucleon cross section we have analyzed the cosmic ray spectra observed at AGASA and HiRes taking also into account results from horizontal events at AGASA and contained events at RICE. We find that scenarios of strongly interacting neutrinos are still compatible with the data requiring a steep increase of the inelastic neutrino-nucleon cross section by four order of magnitude within one energy decade compared to the Standard Model predictions. We also discuss the impact of the preliminary cosmic ray spectrum observed by the Pierre Auger Observatory.