Study of the nature of ultra high energy cosmic rays using the first data of the Pierre Auger Observatory

Due to their weak flux, the existence of the ultra high energy cosmic rays (UHECR, E>10¹⁸ eV) has been and still remains an enigma for more than half a century. Indeed, neither the origin, nor the nature, nor the maximum energy they can reach, are known. To have sufficient statistics and to answer these questions, the Pierre Auger Observatory is currently built in Argentina and will be completed by a second detector in Colorado to observe the northern hemisphere sky. As the origin and the nature of the UHECR are related, the identification of the primary is an important step to validate, refute or constrain theoretical models of UHECR production. In particular, the presence of photons or neutrinos can be the signature of models implying new physics (topological defects, super-heavy particles). In this framework, the work presented in this thesis consists in trying to identify the nature of UHECR using Pierre Auger data since January 2004. After having recapitulated the results of the preceding experiments and having presented the methods of detection, this manuscript describes the theoretical models by specifying the particle type of high energy which they can produce. The following part deals with various methods used by the observatory to allow the identification of the primary particle by the shower created in the atmosphere. Various criteria are finally tested on simulations and then used in an analysis which makes it possible to estimate the hadron composition and to investigate the presence of photons in the UHECR. (author)