EPOS Model and Ultra High Energy Cosmic Rays

The interpretation of extensive air showers (EAS) experiments results is strongly based on air shower simulations. The latter being based on hadronic interaction models, any new model can help to understand the nature of cosmic rays. The EPOS model, reproducing all major results of existing accelerator data (including detailed data of RHIC experiments), has been introduced in air shower simulation programs CORSIKA and CONEX a few years ago. The new EPOS 1.99 has recently been updated taking into account the problem seen in EAS development using EPOS 1.61. We will show in detail the relationship between some EPOS hadronic properties and EAS development, as well as the consequences on the model and finally on cosmic ray analysis.