Proton energy loss in photohadronic interactions on the microwave nackground

Photomeson production is the main energy loss process for relativistic nucleons in dense radiation fields like the cosmic microwave background. In this paper we study the energy evolution of cosmic rays due to pion photoproduction losses in the 2.7K-background radiation field traveling over cosmological distances. We give the median energy of a proton at source as a function of its detected energy together with its energy fluctuations.