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Benjamin Rouillé d'Orfeuil (2004-2007)

Source search and anisotropy studies of cosmic rays in the Pierre Auger Observatory. (ManuscriptDefence)

The study of the extremely high energy cosmic rays (EHECR) gives rise to numerous experimental and theoretical efforts. The origin of these particules of macroscopic energy - a few tens of Joules - is still not identified and is subject to many discussions. The search for the EHECR sources or structures on the sky associated to these sources is one of the main objectives of the Pierre Auger Observatory (PAO) on duty since January 2004. We use the PAO data to search for anisotropies on small and large angular scales. The algorithms we developped for this purpose rely on the accurate knowledge of the sky covered by the detector. It is the average distribution of particles expected if EHECR flux was isotropic. As the probability to detect a cosmic ray in one of the directions of the sky is not constant with time, a detailed study of the temporal variation of the events flux recorded by th PAO is required to correctly estimate the coverage map. In particular, the developpment of extensive air shower triggerd by a EHECR is found to be affected by the weather conditions. All these studies represent an important part of this work, which is completed by the measurement of the EHECR spectrum corrected by atmospheric effects. First of all, we discuss the phenomenology of EHECR, sum up the results obtained by previous experiments and describe the experimental techniques related to the extensive air shower detection.

Eric Armengaud (2003-2006)

Propagation and arrival direction distribution on the sky of UHECRs within the Pierre Auger Observatory (Manuscript)

The origin of ultra-high energy cosmic rays remains an enigma of modern physics, which the Pierre Auger Observatory, a detector with a hybrid detection mode and an unprecedented size, will try to solve. The direct observation of the sources of those particles, or of large-scale structures in the sky associated to the sources, is one of the main goals of the observatory. Such observations should also allow to constrain cosmic ray propagation between their sources and the Earth, which is complicated by interactions with low-energy photon backgrounds and deflections in astrophysical magnetic fields. This thesis is made of two parts, in order to observe and modelize the sources of cosmic rays within the Auger Observatory. We begin with an extensive description of the Pierre Auger Observatory, and study the acceptance of its surface detector in order to build accurate sky exposure maps, an essential tool in order to study anisotropies. Then we present methods to search for anisotropies in the sky, and analyze the first two years of Auger data. After a description of the phenomena that can influence the propagation and observation of ultra-high energy cosmic ray sources, we present numerical simulations aiming at predicting observables such as the spectrum, anisotropies and composition measurable by Auger as a function of various astrophysical models. We show that extragalactic magnetic fields can play a crucial role in particular if cosmic rays are partly heavy nuclei. Finally, we show that the propagation of these particles from a nearby source generates secondary fluxes of gamma-rays that could be detected by TeV gamma-ray telescopes.

Gilles Maurin (2002-2005)

Study of high energy cosmic ray nature with the Auger observatory (Manuscrit, Presentation)

Due to their weak flux, the existence of the ultra high energy cosmic rays (UHECR, E>10^18eV) 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.

Fabrice Cohen (2000-2003)

Atmospheric shower simulations and lateral distribution function at Auger energies (Manuscrit et Presentation)

The Pierre Auger Observatory is the largest experiment ever designed to detect ultra high energy cosmic-rays. Its large area must allow to obtain statistics for energies above 10^19 eV to understand their origin. The Hybrid mode of detection provide accuracy on primary energy reconstruction and on the arrival direction. In the first chapter, we present the history of cosmic rays and the problems occuring at ultra high energy. In a second part, the detectors of the Pierre Auger Observatory are presented, with motivations and constraincts of the installation on the site. In the third part, we describe the extensive air shower simulation. We begin by the first interaction at high energy where hadronic models are unknown and have to be extrapolated. Then, we continue the development of the shower with the electromagnetic cascade and the penetrating component which are detected respectively by fluorescence and surface detectors. The chapter four is dedicated to a new lateral distribution function (decreasing of signal in function of distance to the shower core) that we propose. The new function is able to fit the individual shower behaviour and allows a better reconstruction of energy estimators. we test this new function to the engineering array data. Finally, we present the data of engineering array installed in the Pampa and the reconstruction of its events.