Hadronic Interactions for High Energy Cosmic Ray Air Showers
  Understanding hadronic interactions at high energies represents a very important ingredient for modeling high energy cosmic ray air showers. In this paper, we present a new approach to simulate hadronic interactions, including hadron-hadron, hadron-nucleus, and nucleus-nucleus scattering, in the energy range roughly between 10^11 and 10^17 eV. Such collisions are very complex, being composed of many components, and therefore some strategy is needed to construct a reliable model. The central point of our approach is the hypothesis, that the behavior of high energy interactions is universal (universality hypothesis). So, for example, the hadronization of partons in nuclear interactions follows the same rules as the one in electron-positron annihilation ; the radiation of off-shell partons in hadronic collisions is based on the same principles as the one in deep inelastic scattering. We construct a model for hadronic interactions in a modular fashion. The individual modules, based on the universality hypothesis, are identified as building blocks for more elementary interactions (like e^+ e^-, lepton-proton), and can therefore be studied in a much simpler context. With these building blocks under control, we can provide a quite reliable model for nucleus-nucleus, hadron-nucleus and hadron-hadron scattering, providing in particular very useful tests for the complicated numerical procedures using Monte Carlo techniques. 
H.J. Drescher, M. Hladik, S. Ostapchenko, K. Werner