Highest Energy Cosmic Rays, Grand Unified Theories, and the Diffuse Gamma-Ray Background

We explore scenarios where the highest energy cosmic rays (HECR) are produced by new particle physics near the grand unification scale. Using detailed numerical simulations of extragalactic cosmic and gamma-ray propagation, we show the existence of a significant parameter space for which such scenarios are consistent with all observational constraints. An average fraction of about 10% gamma-rays in the total cosmic ray flux around 10 EeV [10**(19) eV] would imply both a non-acceleration origin of HECR and a large scale extragalactic magnetic field smaller than about 10**(-11) G. Proposed observatories for ultra-high energy cosmic rays should be able to test for this signature.