  We explore scenarios where the highest energy cosmic rays are produced by new
particle physics near the grand unification scale. Using detailed numerical
simulations of extragalactic nucleon, gamma-ray, and neutrino propagation, we
show the existence of an interesting parameter range for which such scenarios
may explain part of the data and are consistent with all observational
constraints. A combination of proposed observatories for ultra-high energy
cosmic rays, neutrino telescopes of a few kilometer scale, and gamma-ray
astrophysics instruments should be able to test these scenarios. In particular,
for neutrino masses in the eV range, exclusive neutrino decay modes of
superheavy particles can give rise to neutrino fluxes comparable to those
predicted in models of active galactic nuclei.
