Search for ultra-high energy photons with the AMIGA muon detector

Cosmic rays (E > 10 eV) have been known for more than 70 years, and yet their origin remains elusive. Similarly, the possibility of a very-high energy photon component of the cosmic radiation is one of the open problems in Astroparticle Physics. The search for high energy photons complements measurements of cosmic rays and neutrinos towards a multi-messenger understanding of the most energetic astrophysical phenomena. In particular, the discovery of photons with energies between 10 eV and 10 eV in the cosmic rays flux could be of particular interest not only for the field of Astroparticle Physics, but also for Astrophysics and fundamental Physics, since they are tracers of the highest-energy processes in the Universe. In the search for ultra-high energy photons, it is crucial to define composition-sensitive parameters capable of adequately rejecting the hadronic cosmic-ray background. The muon content of the extensive air showers produced by primary cosmic rays as they enter the atmosphere is one of the most promising aspects that could lead to the best possible discrimination between photons and hadronic cosmic rays. The AMIGA underground muon detector, as a part of the upcoming AugerPrime upgrade for the Pierre Auger Observatory, offers a unique and straight-forward opportunity to directly measure high-energy muons of extensive air showers, and thus, enhance the sensibility of the Observatory to a primary photon signal. The Pierre Auger Collaboration has proposed several parameters in order to study a possible ultra-high energy photon component in the hadronic cosmic-ray flux. However, the nonobservation of photon candidates resulted in upper limits for energies above 10 eV. On the other hand, the energywindowbetween 10 eV and 10 eV has only been explored by the KASCADEGrande and the EAS-MSU experiments which impose upper limits to the photon flux. Currently, this energy domain is not explored by the Pierre Auger Observatory. The main objective of this thesis is the extension of the ultra-high energy photon search down to ∼ 10 eV. The stringent theoretical and experimental upper limits to the photon flux at these energies make the search of a weak photon signal in the vast hadronic cosmic-ray background a challenging task. Therefore, parameters sensitive to the predominantly electromagnetic signal from photon primaries are of paramount importance. In this framework, we define and describe two new composition-sensitive observables designed for the photon/hadron discrimination quest in order to either detect photon primaries in the 10 eV energy domain or improve the upper limits established by previous experiments. The observable Mb combines the muon densities measured by the AMIGA underground muon stations and their distance to the shower axis, similarly to the well-known Sb parameter used previously in photon searches by the Pierre Auger Collaboration. On the other hand, the observable Q exploits the difference in the slope of the lateral distribution of particles between photonand hadron-initiated showers. In the latter case, the showers are expected to develop higher in the atmosphere and thus they arrive to the groundwith a flatter distribution of particles.