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Interaction on cosmic background

We'll only describe here the UHECRs interaction during their propagation, that is energy lost processes, essentially due to dense photon fields in large scale univers. We'll present interactions with nucleons, nucleus and photons.
Le photon case is special because a single ultra high energy photon generate quickly all of an electromagnetic shower whose spectrum spread down to low energies.
Moreover, we'll assume here that univers is transparent to neutrinos. UHE neutrino propagation, over cosmological distances, is responsible for the flavor mixing. This means that, whatever the nature of the neutrino at its source is, we expect the same number of the three flavors arriving on earth. This explain the interest we have in the Auger experiment for the tau neutrino detection.

Nucleon interaction

High energy protons and neutrons interact with the Cosmological Microwave Background (CMB) and, in a lesser way, with the Cosmological InfraRed Background (CIB) by pair production and meson photoproduction.

Nucleus interaction

Nucleus interact, as nucleons do, with CMB to produce e+e- pairs and pions (by photoproduction). For a nucleus (Z, A, E), the energy loss by pair production is about Z2/A times larger than for a proton of energy E/A. The minimum of LLoss, at the energy of 2Z x 1019 eV, is greater than the one of a nucleon alone. It's the same phenomena for the pion photoproduction : the pertinent factor to estimate the thresold being the Lorentz factor, the threshold energy of this reaction is move to higher energies. The pion photoproduction cross section for the nucleus behave rather like the geometrical factor associated to the nucleus size A2/3.

Electromagnetic cascades

Ultra high energy photons, as well as electrons and positrons, develop in the intergalactic space electromagnetic showers with a mean energy spectrum decreasing with distance covered.
The processes liable to generate photons or e+e- are : The electromagnetic shower, whose energy ranges from ultra high energies to the Gamma Astronomy ones, has a development wich can be described as the following : photons produce e+e- pairs on low energy diffuse cosmological backgrounds; in parallel, e+e- interact by inverse Compton diffusion on the same bacgrounds and can also have a synchrotron emission when they go through magnetic fields. So, shower can develop over quasi cosmological scales, like the electromagnetic component of the EAS (Electromagnectic Air Showers).