It is proposed that the highest energy ∼ 10 20 eV cosmic ray primaries are protons, decay products of a long-lived progenitor whose high kinetic energy arises from decay of a distant (cosmological) superheavy particle, G. Such a scenario can occur in e.g. SU (15) grand unification and in some preon models, but is more generic; if true, these unusual cosmic rays provide a window into new physics...
