A search for photons of energy above 6 × 10¹⁸ eV using data from the Water-Cherenkov detectors of the Pierre Auger Observatory

It is important to develop methods to search for photons in the ultra-high energy cosmic ray beam. Photons above 6 × 10^18 eV can be created if the highest energy cosmic rays are dominated by light nuclei (such as protons) but not if the primaries are dominated by heavy nuclei such as iron. In this thesis a new method is developed to attempt to distinguish showers initiated by photons from those initiated by hadrons. Data from January 2004 to March 2014 that have zenith angle 0 < θ < 60 ̊ are used in the analysis. This is the first time that a parameter based on the surface detectors from the Pierre Auger Observatory is valid for the entire zenith angle range and even for energies below 10^19 eV. 81, 9, 2, 0 and 0 photon-like events were found for the energies above 10^18.8, 10^19, 2 × 10^19, 3 × 10^19 and 4 × 10^19 eV respectively (all photon candidates are considered as hadronic background). The upper limit (95% c.l.) of the photon flux compared to previous analyses that were obtained by other methods has been improved by a factor of 3.