Analysis of Air Showers with respect to Primary Composition of Cosmic Rays

The ultra-high energy (above 10^18 eV) cosmic rays are still of unknown origin and uncertain mass composition; especially at the highest energies. The precise determination of mass composition of cosmic rays is crucial ingredient to better understand these mysterious particles. The methods developed during my Ph.D. study to extend the current knowledge about mass composition of cosmic rays with current and future observatories will be presented. In one case, an application to the data of the Pierre Auger Observatory will be shown. This multivariate analysis indicates a mixed composition of cosmic rays in a wide range of energies. Simultaneously, it provides an estimation of inconsistencies between the measured ground signal and the ground signal simulated using different models of hadronic interactions.