Their origin is unknown. According to various theoretical models, these particles could be nucleons, heavy nuclei, photons, neutrinos, or even more exotic particles. The particle that decays is an hypothetic super-massive particle, that could be associated with grand unification scale physics. It could have been produced in the early Universe, and remained stable until now, or its stability could have been enforced by confinement inside a topological defect. The main problem with understanding the origin of ultra-high energy cosmic rays is the following.
Similar considerations apply to photons or heavy nuclei.
Although the existence of these fields is still to be proven, there is evidence for their presence, notably the observation of a bridge of synchrotron emission connecting two clusters of galaxies in the Coma Supercluster. This work has been done in collaboration with Guenter Sigl , Angela Olinto and David Schramm from The University of Chicago. We found that such a magnetic field would allow to reproduce very nicely the observed energy spectrum of ultra-high energy cosmic rays, through the interplay of energy losses and diffusion in the magnetic field. Rev. D56, 4470. Rev. D52, 1955.