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.
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.
- Pair production
In this process, a high energy proton is involved in the transformation of a low energy photon in a e+e- pair.
In an interaction with a CMB photon whose mean energy is near 10-3,
the reaction threshold is around 5 x 1017 eV.
In fact, only the CMB is important for this interaction. Pair production is the only important loss energy process, dominant for sub-GZK processes. Loss energy distance is minimal around de 2 x 1019 eV and stay always at more than 1 Gpc.
- Pion photoproduction
The pion photoproduction is responsible for the famous GZK supress.
This is the interaction of a proton or a neutron on a CMB photon producing a nucleaon and a pion. The reaction threshold is around 40 EeV (a little less when the interaction invilve a cosmological infra-red background photon).
The cross section associated to this reaction is well known and has been measured by experiments on accelerators, where photons interact with the target's protons. The cross section is maximal (~ 500 micro-barn) at the Delta+ (1230 MeV) resonance mass.
The pion photoproduction, in particular on the CMB photons, is the most important interaction for the high energy nucleons. It reduces the particule's horizon from several hundreds of Mpc at 30 EeV to twenty or so Mpc at 200 EeV, reducing, by consequence, the possible sources of high energy particules.
- Neuton desintegration
Neutrons, being neutal, are, in principle, not accelerated in UHECR sources.
They can, nevertheless, be created by pp interaction near these sources. Furthermore, they are generated in the proton pion photoproductions. They desintegrate into proton, neutrino and electron, which constitutes a new source of neutrinos and electromagnetic secondary particules at ultra high energy.
However, the desintegration distances are relatively short : 9 kpc at 1Eev and 1Mpc at 100 Eev. So, there is little hope to observe an UHE astrophysical source by its neutron componant.
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.
The dominant interaction for the nucleus with trans-GZK energies is the photodesintegration. That means the absorption of a photon, generating an instable state, then the fast emission of one or several nucleons when the photon energy, compared to the nucleus one, is of the order of magnitude of the liaison energy of its nucleons. For a center of mass energy of 10 - 30 MeV, this interaction is dominated by the giant dipolar resonance (GDR). At higher energies (up to ~ 150 MeV), energy losses (with several nucleons emission) is also to be considerated.
In conclusion, the nucleus photodissociation leads to the prediction of a spectral cut, similar to the GZK effect for the nucleons. This consolidate the GZK effect prediction for the UHECR astrophysical models.
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).
- Pair photoproduction of protons on the CMB generate naturally e+ e- pairs.
- Pion photoproduction generate e+ e- via p+- chanel and also gammas by pi0 desintegration.
- Neutron desintegration create electrons
- One can also mention more speculative models pedicting a direct production of an important quantity of photons.