Study of composition and hadronic interactions in ultra-high energy cosmic rays using hybrid data of the Pierre Auger Observatory

Cosmic rays are energetic particles from various, often unknown, astrophysical sources. Upon hitting the atmosphere they produce billions of secondary particles that on their way cause emission of the fluorescence light in air. Many particles reach the ground and their spatial and temporal distributions can be measured with particle detectors. The Pierre Auger Observatory is a hybrid detector where fluorescence radiation is detected using dedicated telescopes, and the distribution of particles at the ground is sampled using water-Cherenkov stations. The information from both types of detectors can be used to estimate the mass of the primary nucleus. Recent results obtained with a novel method using the correlation between information from both detectors for the first time provided reliable evidence on the presence of several different components in the primary radiation and helped to constrain astrophysical scenarios predicting a presence of a single component only. This method now should be extended to a wider range of energies to guide the astrophysical models of production and acceleration of cosmic rays. Additionally, a study of fine details of the physics behind the observed correlation between different observables might help in the understanding of the drawbacks of the hadronic interaction models used for the modelling of particle cascades.