Abstract
The main objective of the project is the determination of the mass composition of ultra-high-energy cosmic rays (UHECRs) with strongly reduced systematic uncertainties from the description of hadronic interactions using the Phase I data of the Pierre Auger Observatory. The accurate determination of the mass composition is the key to understanding the origin of the features (second knee, ankle, instep, suppression) of the UHECRs energy spectrum and to constraining the parameters of the post-LHC hadronic models. Achieving a reliable description of hadronic interactions is required for making possible an event-by-event determination of the mass of primary particles using full statistics of the surface detector of AugerPrime, the upgrade of the Observatory and the mass composition enhanced studies of arrival directions of UHECRs. Such studies might lead to a discovery of the UHECRs sources or allow us to constrain the characteristics of the source candidates and propagation scenarios.