Probing grand unified theories with cosmic-ray, gamma-ray, and neutrino astrophysics - art. no. 043504.

We explore scenarios where the highest energy cosmic rays are produced by n ew particle physics near the grand unification scale. Using detailed numeri cal simulations of extragalactic nucleon, gamma-ray, and neutrino propagati on, we show the existence of an interesting parameter range for which such scenarios may explain part of the data and are consistent with all observat ional constraints. A combination of proposed observatories for ultra-high e nergy cosmic rays, neutrino telescopes of greater than or similar to few ki lometer scale, and gamma-ray astrophysics instruments, should be able to te st these scenarios. In particular, for neutrino masses in the eV range, exc lusive neutrino decay modes of superheavy particles can give rise to neutri no fluxes comparable to those predicted in models of active galactic nuclei . [S0556-2821(99)06202-5].