Search for cosmogenic photons in the ultra high energy cosmic ray flux

This thesis manuscript present two contributions to the study of ultra-high energy cosmic rays with the Pierre Auger Observatory: on one hand the assembly and characterization of scintillation detectors and on the other, the development of a new multivariate analysis for the search of ultra-high energy photons. The scintillation detectors are part of the AugerPrime project, to enhance the performances of the Pierre Auger Observatory. Scintillation detectors are being installed on top of the water Cherenkov detectors making up the surface detector of the observatory. This new system will perform a complementary measurement of the secondary particles inside the atmospheric showers, thereby increasing the capacity of the observatory to identify the nature of the cosmic rays. The LPSC has had the responsibility of assembling 90 detectors. A detailed procedure was written to ensure a continuity throughout the assembly process. Once assembled, the integrity of the detectors is controlled and the performances of each detector are evaluated. This is done by measuring the response of the detectors to a vertical through-going muon in units of photoelectrons. The measurements performed on the detectors assembled at the LPSC are presented: they are compatible with the quality criteria required by the collaboration. They are also compared to the measurements performed in the other assembly sites. The main contribution presented in this manuscript concerns the search for ultra-high energy photons. Indeed, identifying photons in the flux of ultra-high energy cosmic rays would allow a direct localization of cosmic rays sources in the nearby universe. The upper-limits on the flux of such photons, set by the previous analyses have already constrained ultra-high energy cosmic rays production scenarios, such as the so-called 'top-down' scenarios and could constrain the astrophysical scenarios of production and propagation (GZK effect). The analysis presented in this manuscript uses the signals of the water Cherenkov detectors making up the surface detector of the Pierre Auger Observatory in order to construct discriminant observables. These observables take advantage of the differences between hadronic and electromagnetic showers in terms of their muonic component ratio, and in terms of the thickness of their shower fronts. The observables are reconstructed using machine-learning models, trained on simulated events in a biased way in order to differentiate the photons from other types of cosmic rays. Finally, the discriminant observables are combined in a photon search analysis, applied to a sample of data from the Pierre Auger Observatory. The performances of such an analysis are discussed, as well as the perspectives in the framework of AugerPrime. (author)