Cosmic rays as interface between astrophysics and particle physics

The interpretation a extensive air shower (EAS) observations needs a sufficiently accurate knowledge of the interactions driving the cascade development in the atmosphere. While the electromagnetic and weak interaction parts do not provide principal problems, the hadronic interactions is a subject of uncertainties and debates, especially in the ultrahigh energy region extending the energy limit of man made accelerators and experimental knowledge from collider experiments. Since the EAS development is dominantly governed by soft processes, which are presently not accessible to a perturbative QCD treatment, one has to rely on QCD inspired phenomenological interaction models, in particular on string-models based on the Gribov-Regge theory like QGSJET, VENUS and SIBYLL. Recent results of the EAS experiments KASCADE are scrutinized in terms of such models used as generators in the Monte Carlo EAS simulation code CORSIKA. (author)