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Atomic nuclei, photons and neutrinos rain down continuously on the Earth from space. At the Pierre Auger Observatory in Argentina researchers study these high-energy particles with unprecedented precision – and hope to uncover some of the universe's secrets

Atomkerne, Photonen und Neutrinos regnen unablassig aus dem Weltraum auf die Erde herab. Am Pierre Auger-Observatorium in Argentinien blicken Forscher mit bisher unerreichter Prazision auf die hochstenergetischen Teilchen – und erhoffen sich dadurch neues Wissen uber unser Universum

In this paper, we will discuss the prospects of deducing the proton-air cross section from fluorescence telescope measurements of extensive air showers. As it is not possible to observe the point of first interaction $X_{\rm 1}$ directly, other observables closely linked to $X_{\rm 1}$ must be inferred from the longitudinal profiles. This introduces a dependence on the models used to describe the shower development. The most straightforward candidate for a good correlation to $X_{\rm 1}$ is the depth of shower maximum $X_{\rm max}$. We will discuss the sensitivity of an $X_{\rm max}$-based analysis on $\sigma_{\rm p-air}$ and quantify the systematic uncertainties arising from the model dependence, parameters of the reconstruction method itself and a possible non-proton contamination of the selected shower sample., Comment: 4 pages, Proceedings for ISVHECRI Weihei 2006

The intensity of cosmic rays is steeply falling with energy. The differential spectrum can be approximated by an E −α power law. It shows some distinct features. The change in the spectral index α from 2.7 to 3.1 occurring at 3 × 10 15 eV is called the knee. Another spectral break occurs at 3 × 10 18 eV, the ankle, where the spectru mt urns up again. The highest energies observed so fa rr each 3 × 10 20 eV. This observation is a major puzzle, and the subject is the focus of intense research. Particles beyond the ankle are thought to be of extragalactic origin because their Lamor radius is larger than the size of the galaxy. Due to interactions with th e3 Kc osmic microwave background photons a sharp cut-off is expected at about 5 × 10 19 eV, named after Greisen, Zatsepin and Kuz’min (GZK). The existence of the GZK cut-off in contrast to a continuing spectrum is currently under investigation. Different models for the origin of the ultrahigh-energy events have been designed. Some of them are able to reproduce observed features, some of them are highly speculative, and all are awaiting some experimental validation. The nature, origin and propagation of cosmic rays are still a mystery 90 years after their discovery by Hess and Kohlh¨ orster. This paper focuses on the experimental situation regarding cosmic rays at the highest energies. More than 60 years after Pierre Auger initially recorded and correctly interpreted extended air showers, this field is more active than ever before. Existing data from the experiments performed so far will be reviewed and put into context. Current challenges will be discussed with the perspectives coming from new projects, in particular from the Pierre Auger Observatory in Argentina. (Some figures in this article are in colour only in the electronic version)

This contribution very briefly summarizes the results on the nature and origin of ultra‐high energy cosmic rays obtained with the Pierre Auger Observatory. On that basis the motivation for a second observatory providing significantly larger instrumented area, high‐quality shower measurements and full‐sky coverage is presented.

The aim of the EDELWEISS-III experiment is to detect the elastic scattering of WIMPs from the galactic dark matter halo on germanium bolometers. The most problematic background arises from neutrons, which can mimic a WIMP interaction in a detector. Neutrons are notably induced by high energy cosmic ray muons reaching the underground laboratory despite the 4800 m w.e. of rock overburdened. Remaining muons are tagged using an active muon-veto system of 46 plastic scintillator modules surrounding the experiment, which allows to reject most of the associated background. The goal of this thesis was to give a precise estimation of the irreducible muon-induced neutron background, needed to identify a potential WIMP signal. The expected background depends on the geometry of the experiment as well as on the used materials, both strongly modified since EDELWEISS-II. Geant4-based simulations of muons through the modified geometry were performed to derive the rate of events induced by muons in the bolometer array. This rate has been shown to be in good agreement with the measured one extracted from the Run308 data. In parallel, a lower limit on the muon-veto efficiency was derived using bolometer data only. A new method based on an AmBe source was developed to extract precisely the detection efficiency of individual modules from the simulation. From these results, it was shown that the expected background is negligible for the WIMP search analyses performed with the Run308 data and won't limit the future sensitivity of the EDELWEISS-III experiment, Le but de l'expérience EDELWEISS est la détection directe de matière noire sousforme de WIMPs, par l'étude de leur diffusion élastique sur les noyaux de germanium des détecteurs bolomètriques. Le plus problématique des bruits de fond provient des neutrons pouvant mimer l'interaction d'un WIMP dans un détecteur. Ces neutrons sont notamment produits par les rares muons cosmiques de haute énergie qui atteignent le laboratoire souterrain malgré les 4800 m w.e. de roche. Les muons résiduels sont détectés par un système veto de 46 modules de scintillateur plastique entourant l'expérience, qui permet de rejeter la plupart du bruit de fond associé. La détermination précise du bruit de fond neutron résiduel induit par ces muons dans EDELWEISS-III, essentielle pour l'identification des WIMPs, est le but de cette thèse. Le taux de bruit de fond dépend de la géométrie de l'expérience ainsi que des matériaux utilisés, qui ont subi d'importantes modifications depuis EDELWEISS-II. Des simulations GEANT4 du passage des muons dans la nouvelle géométrie ont été réalisées afin d'extraire le taux d'événements induits par les muons dans les bolomètres. Ce taux est en bon accord avec le taux mesuré extrait des données du Run308. En parallèle, une limite inférieure sur l'efficacité du veto muon a été extraite à partir des données bolomètres. Une nouvelle méthode basée sur l'utilisation d'une source d'AmBe a été développée afin d'extraire l'efficacité de chaque module de la simulation. À partir de ces résultats, il a été montré que le bruit de fond attendu est négligeable pour la recherche de WIMPs avec les données du Run308 et ne limitera pas la sensibilité future d'EDELWEISS-III

The EDELWEISS experiment aims to detect WIMPs, weakly interactive massive particles, which could possibly amount for all or part of the dark matter in the universe. It measures the energy released by nuclear recoils produced by the elastic collision of a WIMP in an ordinary matter target. Due to the very small interaction cross-section of WIMP with nucleons, which leads to an extremely low expected event rate (< 1 event/kg/year), and due to the relatively small deposited energy (< 100 keV), the nuclear recoil signal of WIMP events can be mimiced by neutrons coming from natural radioactivity or induced by muons. The present work is devoted to study the muon-induced background. Performances of the muon veto of the EDELWEISS-II experiment are presented and detection of muons and showers discussed. The first muon veto-bolometers coincidences has been performed on two 4-month physics runs in 2007 and 2008 and leads to a result of 0:043 § 0:015 coinc/kg/d for a recoil energy ER