Particle dynamics and GZK limit in relativity with a preferred frame

The theory termed ‘special relativity (SR) with a preferred frame’ incorporates the preferred frame into special relativity while retaining the fundamental space-time symmetry which, in the standard SR, manifests itself as Lorentz invariance. In this paper, the theory is extended to particle dynamics based on the concept of the ‘modified spacetime symmetry’ which allows to develop the theory along the lines of the standard relativity theory. The modified relativistic dynamics is applied to describing the interactions of the Ultra High Energy Cosmic Rays (UHECR) with universal diffuse background radiation and, in particular, to calculating the Greisen–Zatsepin–Kuzmin (GZK) threshold for the energy suppression due to pion photoproduction by UHECR protons. The result, that the GZK cutoff energy depends on the distance (cosmological redshift z) to the source of particles, may contribute to interpretation of the data on the mass composition of UHECR.