Há várias evidências experimentais da existência de matéria escura no universo. Apesar destas evidências, pouco se sabe sobre sua constituição, sabendo-se apenas que interage gravitacionalmente, mas não eletromagneticamente. Neste projeto, investigamos a possibilidade da matéria escura ser composta por partículas supermassivas e fortemente interagentes (Simpzillas). Para isto determinamos o sinal que deve ser deixado no telescópio IceCube por neutrinos resultantes de aniquilações de matéria escura no Sol. Após determinarmos o espectro de neutrinos no centro do Sol, simulamos sua propagação até a superfície, depois até a Terra e através da Terra até o detector. Comparamos então estes resultados com os fornecidos pelo IceCube. Esta comparação permite testar uma região do espaço de fase massa versus seção de choque previamente não-excluída por outros tipos de experiência que não telescópios de neutrinos. Como resultado, concluímos que partículas supermassivas e fortemente interagentes não podem constituir a matéria escura. / There are many experimental evidences for the existence of dark matter in the universe. Despite these evidences, there is no knowledge about its constitution other than the fact that it interacts gravitationally but not electromagnetically. In this project, we investigate the possibility that dark matter is composed of strongly interacting massive particles (Simpzillas). We determine the expected signal in the IceCube telescope from Simpzilla annihilation in the center of the Sun. We first determine the neutrino spectrum in the core of the Sun. We then simulate its propagation through both the Sun and Earth, and finally the rate of neutrinos at the detector. A comparison of these results to the ones published by the IceCube collaboration covers a large region of the yet not excluded regions of the mass versus cross-section phase space. As a result, the possibility of Simpzillas composing the dark matter is ruled out.

A determinação da massa de raios cósmicos ultra energéticos é uma questão em aberto devido a flutuação natural da primeira interação e o desconhecimentos da seção de choque para altas energias (E > 1017 eV). Alguns estudos (1) mostraram que modificações sistemáticas das propriedades em interações hadrônicas interfere significativamente no desenvolvimento de chuveiros atmosféricos extensos e, consequentemente, provêm diferentes interpretações para os resultados de raios cósmicos. No limite de altas energias, pode ocorrer a saturação de glúons na superfície nuclear, o que resulta no aumento da seção de choque próton-próton e próton-núcleo em relação ao modelo de Glauber. A seção de choque cresce com a energia e está relacionada com a função de distribuição de probabilidade de encontrar um centro espalhado que é maior no modelo de saturação de glúons. Neste trabalho analisamos a seção de choque próton-ar e núcleo-ar para energias acima de 1018 eV usando a hipótese da saturação de glúons (2). Implementamos a saturação de glúons no modelos de interações hadrônicas de altas energia SIBYLL e estudamos o desenvolvimento do chuveiro atmosférico extenso usando o programa CORSIKA. Comparamos os resultados com a taxa de elongação medida pelo Observatório Pierre Auger. Concluímos mostrando o efeito da saturação de glúons na interpretação na composição da massa de raios cósmicos ultra energéticos. / The determination of the composition of ultra high energy cosmic rays is an open question due to natural fluctuations of the first interaction point and unknow high energy cross sections. Some studies (1) have shown that systematic modifications in the hadronics interaction properties infer significantly in the development of extensive air showers and, consequently, provides different interpretations of cosmic ray results. In high energy limit (E > 1018eV), gluon saturation may occur in the nuclear surface region, so that the cross section proton-proton and proton-nucleus increase more rapidly with incident energy than Glauber theory. The cross section rise with energy is related to the probability distribution function of scattering centers which is larger for gluon saturation models. In this work we analyze the cross section proton-proton and proton-nucleus at energies above 1018 eV using the gluon saturation hypothesis (2). We implemented the gluon saturation physics in the SIBYLL hadronic interaction model and we studied the development of the air shower using the CORSIKA program. Different extrapolation models of high energy interaction with gluon interaction have been implemented in SIBYLL. We compared the simulation results using gluon saturation with the measured elongation rate published by the Pierre Auger Collaborations. We conclude by showing the effect of gluon saturation in the interpretation of cosmic ray by composition.

O comportamento das interações hadrônicas para energias de centro de massa superiores a 50 TeV/núcleon é uma fonte importante de incerteza sistemática na interpretação de dados de observatórios de raios cósmicos. Nesta tese, estudamos por meio de simulações de Monte Carlo observáveis dos chuveiros atmosféricos que pudessem ser correlacionados com aspectos dos diferentes modelos destas interações. Especial atenção foi dada à componente muônica que sabidamente é gerada do decaimento de partículas de natureza hadrônica na cascata. A precisão e a acurácia de uma rede esparsa de contadores de múons foram estudadas como função da granularidade da rede de detectores e da área de coleção de suas células. Mostramos, em particular, as diferenças na dependência radial da densidade de múons no plano transversal ao eixo da cascata para os diferentes modelos. Tais diferenças se mostram mais acentuadas na vizinhança do eixo e poderiam ser exploradas experimentalmente com um detector com alcance dinâmico suficiente para evitar saturação da eletrônica. Além disso, por meio de um tratamento fenomenológico, investigamos o impacto no desenvolvimento do chuveiro causado por variações na distribuição de pseudorapidez $dN/d\\eta$ de secundários produzidos na colisão primária. Mostramos que diferentemente de quantidades como seção de choque inelástica, multiplicidade, razão de carga e elasticidade, mudanças em $dN/d\\eta$ que preservem tais quantidades são rapidamente diluídas nas sucessivas camadas de interações posteriores à colisão primária. / The behavior of the hadronic interactions at center of mass energies greater than 50 TeV/nucleon is an important source of systematic uncertainty on interpreting data from cosmic ray observatories. In this thesis, we study through Monte Carlo simulations the atmospheric shower observables that could be correlated with different aspects of these interactions. Special attention was paid to the muonic component that is known to be generated from the decay of hadronic particles. The precision and accuracy of a sparse grid of muon counters were studied as a function of the array granularity and the collection area of its cells. In particular, we studied the differences in the radial dependence of the muon density in the shower plane for different models. These differences are shown to be larger in the vicinity of the shower axis and, therefore, could be explored with a detector of sufficiently large dynamic range as to avoid saturation of its acquisition electronics. In addition, through a phenomenological treatment, we investigated the impact on the shower development caused by variations in the distribution of pseudorapidity $ dN/d\\eta $ of secondaries particles produced in the primary collision. We show that unlike quantities such as inelastic cross section, multiplicity, charge ratio and elasticity, changes in $ dN/d\\eta $ that preserve such quantities are rapidly diluted in successive interaction layers subsequent to the primary collision.

The cosmic rays are the most energetic particles in the universe. Their production, propagation, and detection are objects of studies. Surface detectors aim to identify particles from extensive air showers (EAS) which the result from the cosmic-ray interactions with the atmosphere. Resistive Plate Chambers (RPCs) have shown to be a suitable muon detector to be integrated into the Pierre Auger Observatory. An instrumentation was developed to assembly RPCs in São Carlos (BRA). Data from RPCs already built by our collaborators in Coimbra (POR) were analyzed. The detector efficiency to muons was calculated and is approximately 88%, which is in good agreement with the values quoted in the literature. Direction maps were built to investigate the muon incoming direction and the quantity of matter traversed by the muons. The dependence of the muon flux on the zenith angle was calculated and compared with results from the simulation. A square cosine dependence is expected, though it is seen that the building structure has enough matter to block some of the incident muons and alter the dependence curve. The total muon flux was estimated based on the detector efficiencies and solid angle as 1.6.10−5. mm−2.sr−1. s−1 compared with the literature value of 7.1.10−5 mm−2.sr−1.s−1, which gives an absorption by the building of approximately 77%. / Os raios cósmicos são as partículas mais energéticas do universo. Sua produção, propagação e detecção são objetos de estudos. Os detectores de superfície têm como objetivo identificar partículas dos chuveiros atmosféricos extensos (EAS), o qual é o resultado das interações do raio cósmico com a atmosfera. A Câmaras de Placas Resistivas (RPCs) demonstra ser um detector de múons adequado para ser integrado ao Observatório Pierre Auger. Foi desenvolvida em São Carlos (BRA) uma instrumentação para montagem de RPCs. Dados de RPCs já construídas por nossos colaboradores em Coimbra (POR) foram analisados. A eficiência dos detectores para múons foi calculada como sendo de aproximadamente 88%, o que está de acordo com os valores citados na literatura. Mapas de direção foram construídos para investigar a direção de chegada e a quantidade de matéria atravessada pelos múons. A dependência do fluxo de múons com o ângulo zenital foi comparada com os resultados da simulação. Embora uma dependência com o quadrado do cosseno é esperada, foi constatado que a estrutura do prédio tem matéria suficiente para bloquear parte dos múons incidentes e alterar a curva da dependência. O fluxo total de múons foi estimado baseado nas eficiências do detector e no ângulo sólido é de 1.6.10−5 mm−2.sr−1.s−1. Comparado com o valor da literatura de 7.1.10−5 mm−2.sr−1.s−1 resulta em uma absorção pelo prédio de aproximadamente 77% do fluxo de múons.

A hipótese de que a matéria estranha de quarks (um plasma composto por quarks up, down e strange em quantidades aproximadamente iguais) é estável `a pressão nula vem sendo considerada há mais de vinte anos, tanto teoricamente quanto em experimentos específicos para sua eventual detecção. Se a matéria estranha é de fato estável, então poderia haver importantes implicações para a Astrofísica. Entre as mais estimulantes destas implicações está a possibilidade de conversão da matéria nuclear ordinária em matéria estranha no interior das estrelas de nêutrons devido ás altíssimas densidades ali encontradas. Processos tais como coalescência em binárias de estrelas de nêutrons e as próprias supernovas, eventos responsáveis pelo nascimento destas estrelas, poderiam ejetar estruturas finitas de matéria estranha, chamadas strangelets (equivalentes aos núcleos), no meio interestelar. Desta forma, strangelets estariam presentes entre os primários de raios cósmicos e seriam sujeitas a processos elementares análogos aos núcleos ordinários. Nesta Tese, as strangelets são estudadas desde os prováveis sítios para sua produção astrofísica, passando por interações com a matéria do meio interestelar, ate a chegada ás imediações terrestres. Estima-se o fluxo de strangelets de baixa energia que poderia ser mantido aprisionado na magnetosfera da Terra, bem como são estudados os processos de suas interações com componentes da atmosfera, com vistas às assinaturas observacionais então resultantes. Desta forma, são determinadas as características relevantes para a identificação destes exóticos por experimentos que testam o fluxo de raios cósmicos, ajudando na melhor compreensão das propriedades da matéria nuclear em alta densidade bariônica e baixa temperatura. / The strange quark matter hypothesis, which states that a plasma composed of quarks up, down and strange in roughly equal amounts is absolutely stable at zero pressure, has been studied for more than twenty years, both theoretically and during searches for its detection in specific experiments. If strange quark matter is indeed stable, then there could be important implications for the field of Astrophysics. Among the most stimulating ones is the possibility of conversion of ordinary nuclear matter in strange quark matter in the interior of neutron stars due to the extremely high densities reached in the core of these compact objects. Processes such as the merger in neutron star binaries systems and supernovae themselves, responsible for the birth of these stars, may eject lumps of strange quark matter, termed strangelets, in the interstellar medium. In this way, strangelets may be present among the cosmic ray flux and be subjected to elementary processes much in the same way as ordinary nuclei. In this Thesis, strangelets are studied from their likely astrophysical production sites, passing through the interstellar medium until they reach the Earth neighborhood. Estimates of the low energy flux of strangelets that could be trapped in the terrestrial magnetosphere are given. Also, the interaction of these particles with components in the Earth atmosphere are studied with the aim of providing better understanding of the resulting observational signatures. It allows the determination of the relevant characteristics for the identification of these exotics by experiments testing the cosmic ray flux, helping to better understand the properties of nuclear matter at high densities and low temperatures.

Zur Zeit wird das IceCube Neutrino-Teleskop am Südpol im Eis der Antarktis installiert, die Hälfte des Detektors ist bereits im Betrieb. Bei Fertigstellung im Jahr 2011 wird mehr als 1 km^3 Eis mit Photovervielfachern instrumentiert sein. IceCube bietet damit eine einzigartige Möglichkeit, die Quellen der kosmischen Strahlung mit Hilfe hochenergetischer Neutrinos zu finden. Im Rahmen dieser Arbeit wurde die Sensitivität des kompletten Icecube Detektors für den Nachweis eines diffusen Flusses von Elektronneutrinos bestimmt. Ziel war es, die Eigenschaften des Detektors für Energien oberhalb von einem PeV zu bestimmen. Besonderes Augenmerk wurde dabei auf die Simulation von elektromagnetischen Kaskaden gelegt, die in Neutrino-Nukleon-Wechselwirkungen auftreten. Da existierende Parametrisierungen die Unterdrückung der Wechselwirkungsquerschnitte durch den LPM-Effekt nicht beinhalten, wurde eine Simulation des Energieverlustes von elektromagnetischen Kaskaden für Energien oberhalb von 1 PeV entwickelt, die entsprechend modifizierte Wirkungsquerschnitte verwendet. Die Analyse, die in dieser Arbeit vorgestellt wird, nutzt die komplette Information des durch einen Photovervielfacher aufgezeichneten Ladungsverlaufes aus, die mit der Datennahme des IceCube Detektors zur Verfügung steht. Es werden neue Methoden entwickelt, um zwischen atmosphärischen Myonen-Hintergrund- und Signalereignissen von Kaskaden aus Neutrino-Nukleon-Wechselwirkungen zu unterscheiden. Die erreichbare Sensitivität innerhalb einer Laufzeit von einem Jahr ist 1.5*10^-8 E^-2 GeV/(cm^2 sr s) in einem Energiebereich von 16 TeV bis 13 PeV für den Nachweis von Elektronneutrinos eines diffusen Flusses. Eine Verbesserung von mindestens einer Größenordnung wird erwartet, wenn alle Neutrinofamilien in die Analyse einbezogen werden. Damit sollte eine Sensitivität erreicht werden, die auf dem gleichen Niveau einer diffusen Myonenanalyse liegt. / IceCube is a neutrino telescope currently under construction in the glacial ice at South Pole. At the moment half of the detector is installed, when completed it will instrument 1 km^3 of ice providing a unique experimental setup to detect high energy neutrinos from astrophysical sources. In this work the sensitivity of the complete IceCube detector for a diffuse electron-neutrino flux is analyzed, with a focus on energies above 1 PeV. Emphasis is put on the correct simulation of the energy deposit of electromagnetic cascades from charged-current electron-neutrino interactions. Since existing parameterizations lack the description of suppression effects at high energies, a simulation of the energy deposit of electromagnetic cascades with energies above 1 PeV is developed, including cross sections which account for the LPM suppression of bremsstrahlung and pair creation. An attempt is made to reconstruct the direction of these elongated showers. The analysis presented here makes use of the full charge waveform recorded with the data acquisition system of the IceCube detector. It introduces new methods to discriminate efficiently between the background of atmospheric muons, including muon bundles, and cascade signal events from electron-neutrino interactions. Within one year of operation of the complete detector a sensitivity of 1.5*10^-8 E^-2 GeV/(cm^2 sr s) is reached, which is valid for a diffuse electron-neutrino flux in the energy range from 16 TeV to 13 PeV. Including all neutrino flavors in this analysis, an improvement of at least one order of magnitude is expected, reaching the anticipated performance of a diffuse muon analysis.

Aktive galaktische Kerne (AGN) gehören zu den besten Quellkandidaten der hochenergetischen kosmischen Strahlung. Es wird erwartet, dass hochenergetische Neutrinos durch Interaktion der kosmischen Strahlung mit Materie oder Photonfeldern in der Nähe der Quellen erzeugt werden. Der resultierende Neutrinofluss kann dieselbe Zeitvariabilität aufweisen wie elektromagnetische Strahlung die von diesen Quellen emittiert wird. Diese Zeitvariabiltät kann in Neutrinoanalysen zusätzlich zu Energie-und Ortsinformationen benutzt werden, um die Detektionswahrscheinlichkeit zu erhöhen. Im Rahmen dieser Arbeit werden zwei neue Methoden entwickelt, welche benutzt werden um nach Neutrino-flares in Aktiven Galaktischen Kernen zu suchen: Die Multi-flare und Multi-flare-Stacking-Methode. Die Multi-flare-Methode ist so entworfen, dass sie nicht nur sensitiv auf einen hellen Flare ist, sondern auch auf weitere schwächere Flares welche normalerweise individuell nicht detektiert werden können. Die Multi-Flare-Methode benötigt keine Zeitkoinzidenz mit Ausbrüchen im elektromagnetischen Spektrum. Sie ist auch sensitiv auf unkorrelierter Neutrinoemission mit unterschiedlicher Dauer der einzelnen Flares, was in einigen Emissionsmodellen vorkommt. Die Multi-Flare-Stacking-Methode ist eine Erweiterung der Multi-Flare-Methode auf zusätzliche Quellen. In ihr werden mehrere schwache, variable Quellen, welche individuell zu schwach sein können um detektiert zu werden, zusammen mit der Stackingmethode analysiert. Die beiden Analysemethoden werden auf eine vorselektierte Liste von Aktiven Galaktischen Kernen angewandt. Hierfür werden drei Jahre Daten des IceCube Neutrinoteleskops verwendet (Mai 2009-June-2012). Kein statistisch signfikanter Neutrinoflare wurde gefunden und obere Fluenzgrenzen f ̈ur jede der Quellen werden ausgerechnet. Diese Grenzen sind im Durchschnitt um einen Faktor zwei besser als vorherige Obergrenzen von Analysen einzelner Flares. / Active Galactic Nuclei are among the best candidate sources for high-energy cosmic rays. High-energy neutrinos are expected to be produced in these sources via interactions of cosmic rays with matter or photon fields present in the source vicinity. The resulting neutrino flux may exhibit time variability on the same time scales than the ones observed in the electromagnetic radiation that is emitted from these sources. Time variability can be taken into account in high-energy neutrino searches in order to increase their detection probability with respect to search methods that include only energy and spatial information. In this work, two new methods are developed to look for high-energy neutrino flares emitted from Active Galactic Nuclei: the Multi-flare and Multi-flare stacking method. The Multi-flare method is designed to be sensitive not only to one bright flare emitted from a single source, as considered in other existing search methods, but also to several weak flares that might not be detected individually. This is achieved by developing a likelihood stacking approach that analyzes the cumulative neutrino emission from several flares. This method does not assume a-priori time coincidences with photon flares observed in the electromagnetic spectrum, allowing uncorrelated neutrino emission with different flare durations as considered in some emission models. The Multi-flare stacking method is an extension of the Multi-flare method to include several sources that might be too weak for individual detection. The two search methods are applied to a pre-selected list of Active Galactic Nuclei using data of the IceCube Neutrino Observatory (May-2009 to May 2012). No statistically significant neutrino flares are detected and fluence upper limits are calculated for each selected source. These limits are on average a factor of two better than previous upper limits from single-flare searches.

Les aérosols atmosphériques à l'Observatoire Pierre Auger : caractérisation et effet sur l'estimation de l'énergie des rayons cosmiques d'ultra-haute énergie.L'Observatoire Pierre Auger, situé dans la province de Mendoza en Argentine, réalise actuellement de grandes avancées dans la connaissance de la nature et de l'origine des rayons cosmiques d'ultra-haute énergie. Utilisant une technique de détection hybride, basée sur des détecteurs de surface et des télescopes de fluorescence, il fournit une large statistique, une bonne résolution en énergie, et un contrôle solide des incertitudes systématiques.L'un des principaux défis pour la technique de détection par fluorescence est la compréhension de l'atmosphère, utilisée comme un calorimètre géant. Afin de réduire autant que possible les incertitudes systématiques sur les mesures par fluorescence, la Collaboration Auger a développé un important programme de suivi de l'atmosphère. Le but de ce travail est d'améliorer notre compréhension sur les aérosols atmosphériques, ainsi que leur effet sur la propagation de la lumière de fluorescence.En utilisant un modèle de rétrotrajectographie des masses d'air, il a été montré que les nuits pauvres en aérosols ont des masses d'air provenant plus directement de l'Océan Pacifique. Pour la première fois, l'effet de la taille des aérosols sur la propagation de la lumière a été estimé. En effet, selon l'approche Ramsauer, les gros aérosols ont le plus grand effet sur la diffusion de la lumière. Ainsi, la dépendance en taille a été ajoutée aux paramétrisations décrivant la diffusion de la lumière et utilisée par la Collaboration Auger. Une surestimation systématique de l'énergie et du maximum de développement de la gerbe Xmax est observé.Enfin, une méthode basée sur les tirs laser très incliné produit par le laser central d'Auger a été développée pour estimer la taille des aérosols. Des tailles d'aérosols jusque là jamais détectées à l'Observatoire Pierre Auger peuvent à présent être contraintes. De premiers résultats montrent une population d'aérosols de grande taille en utilisant des tirs laser effectués dans le passé. / Atmospheric aerosols at the Pierre Auger Observatory: characterization and effect on the energy estimation for ultra-high energy cosmic rays.The Pierre Auger Observatory, located in the Province of Mendoza in Argentina, is making good progress in understanding the nature and origin of the ultra-high energy cosmic rays. Using a hybrid detection technique, based on surface detectors and fluorescence telescopes, it provides large statistics, good mass and energy resolution, and solid control of systematic uncertainties.One of the main challenges for the fluorescence detection technique is the understanding of the atmosphere, used as a giant calorimeter. To minimize as much as possible the systematic uncertainties in fluorescence measurements, the Auger Collaboration has developed an extensive atmospheric monitoring program. The purpose of this work is to improve our knowledge of the atmospheric aerosols, and their effect on fluorescence light propagation.Using a modelling program computing air mass displacements, it has been shown that nights with low aerosol concentrations have air masses coming much more directly from the Pacific Ocean. For the first time, the effect of the aerosol size on the light propagation has been estimated. Indeed, according to the Ramsauer approach, large aerosols have the largest effect on the light scattering. Thus, the dependence on the aerosol size has been added to the light scattering parameterizations used by the Auger Collaboration. A systematic overestimation of the energy and of the maximum air shower development Xmax is observed.Finally, a method based on the very inclined laser shots fired by the Auger central laser has been developed to estimate the aerosol size. Large aerosol sizes ever estimated at the Pierre Auger Observatory can now be probed. First preliminary results using laser-shot data collected in the past have identified a population of large aerosols.

Das Energiespektrum der kosmischen Strahlung bietet wichtige Hinweise auf ihren Ursprung und ihre Ausbreitung. Verschiedene Messtechniken müssen kombiniert werden, um den ganzen Energiebereich abdecken zu können: Direkte Messungen mit Teilchendetektoren bei niedrigen Energien sowie indirekte Messungen von Luftschauern bei hohen Energien. Dazu kommt die Messung von Photonen, hauptsächlich im GeV- und TeV-Bereich, die bei der Wechselwirkung von kosmischer Strahlung mit Materie oder elektromagnetischen Feldern entstehen. Im Folgenden werde ich zwei Studien dazu vorstellen, die beide auf Daten des abbildenden Tscherenkow-Teleskopes VERITAS beruhen. Erstens stelle ich eine Messung das Energiespektrums von Eisenkernen in der kosmischen Strahlung vor. Für die Bestimmung der Energie und Ankunftsrichtung der Primärteilchen benutze ich eine neuartige Template-Likelihood-Methode, die hier erstmals auf Eisenschauer angewendet wird. Zur Identifizierung der Eisenschauer benutze ich unter anderem das sogenannte direkte Tscherenkow-Licht, welches von geladenen Teilchen vor der ersten Wechselwirkung ausgestrahlt wird. Dazu kommt eine multivariate Klassifizierungsmethode, um den Verbleibenden Untergrund zu charakterisieren. Das so gemessene Energiespektrum von Eisen wird im Bereich von 20 TeV bis 500 TeV gut durch ein Potenzgesetz beschrieben. Zweitens beschreibe ich eine Suche nach Gammastrahlung oberhalb von 100 GeV von den drei Galaxien Arp 220, IRAS 17208-0014 und IC 342. Diese drei Galaxien haben hohe Sternentstehungsraten und daher viele Supernova-Überreste, welche kosmische Strahlung erzeugen. Diese wechselwirkt erwartungshalber mit den dichten Staubwolken in den Sternentstehungsgebieten und erzeugt Gammastrahlung. VERITAS konnte keine solche Gammastrahlung messen. Die daraus abgeleitete Höchstgrenze für die Luminosität schränkt theoretische Modelle der Erzeugung und Propagation von kosmischer Strahlung in der Galaxie Arp 220 ein. / The energy spectrum of cosmic rays can provide important clues as to their origin and propagation. Different experimental techniques have to be combined to cover the full energy range: Direct detection experiments at lower energies and indirect detection via air showers at higher energies. In addition to detecting cosmic rays at Earth, we can also study them via the electromagnetic radiation, in particular gamma rays, that they emit in interactions with gas, dust, and electromagnetic fields near the acceleration regions or in interstellar space. In the following I will present two studies, both using data taken by the imaging air Cherenkov telescope (IACT) VERITAS. First, I present a measurement of the cosmic ray iron energy spectrum. I use a novel template likelihood method to reconstruct the primary energy and arrival direction, which is for the first time adapted for the use with iron-induced showers. I further use the presence of direct Cherenkov light emitted by charged primary particles before the first interaction to identify iron-induced showers, and a multi-variate classifier to measure the remaining background contribution. The energy spectrum of iron nuclei is well described by a power law in the energy range of 20 to 500 TeV. Second, I present a search for gamma-ray emission above 100 GeV from the three star-forming galaxies Arp 220, IRAS 17208-0014, and IC342. Galaxies with high star formation rates contain many young and middle-aged supernova remnants, which accelerate cosmic rays. These cosmic rays are expected to interact with the dense interstellar medium in the star-forming regions to emit gamma-ray photons up to very high energies. No gamma-ray emission is detected from the studied objects and the resulting limits begin to constrain theoretical models of the cosmic ray acceleration and propagation in Arp 220.

IceTop ist ein Luftschauerdetektor am geographischen Südpol und die Oberflächenkomponente des Neutrinoteleskops IceCube. Seit der Fertigstellung im Dezember 2010 besteht IceTop aus 81 Detektorstationen auf einer Fläche von einem Quadratkilometer. Die vorliegende Dissertation umfasst eine Analyse von Daten, die im Jahr 2007 mit den 26 damals installierten Stationen genommen wurden. Dazu wurden zunächst Schauerposition und -richtung und ein Maß für die Größe des Schauers aus den Signalen rekonstruiert. Der Zusammenhang zwischen Primärenergie und Schauergröße wurde aus Monte-Carlo-Simulationen bestimmt. Dabei wurde eine Annahme über die chemische Zusammensetzung der kosmischen Strahlen gemacht. Damit wurde das Spektrum der Schauergrößen entfaltet um das Energiespektrum zu bestimmen. Dies wurde getrennt für verschiedene Zenitwinkelbereiche durchgeführt. Das Resultat der Entfaltung hängt von der angenommenen Massenkomposition ab. Die Ergebnisse, der Entfaltung in verschiedenen Zenitwinkelintervallen, müssen übereinstimmen, wenn man Isotropie der komischen Strahlung voraussetzt. Es wurde bereits gezeigt, dass dies ausgenutzt werden kann, um die möglichen Annahmen über die Zusammensetzung einzugrenzen. Eine gute Übereinstimmung von Spektren aus verschiedenen Zenitwinkelbereichen wird unter der Annahme von reinen Protonen als Primärteilchen gefunden, sowie für eine Mischung aus Protonen und Eisen mit einem hohen Protonanteil bei niedrigen Energien und einer Mehrheit von Eisen bei hohen Energien. Unter diesen Annahmen ergibt sich die Position des Knies im Spektrum zu 3,97 bis 4,20PeV. Der spektrale Index unterhalb des Knies ist etwa -2,7 und oberhalb des Knies variiert er zwischen -3,08 und -3,15. Reines Eisen auf der anderen Seite kann mit sehr großer Wahrscheinlichkeit ausgeschlossen werden. Unabhängig von der Annahme über die Zusammensetzung wird oberhalb von etwa 30PeV ein Abflachen des Spektrums mit einem Index von etwa -3,0 beobachtet. / IceTop is an air shower array at the geographic South Pole. It is the surface component of the IceCube neutrino telescope. Since its completion in December 2010, IceTop consists of 81 detector stations covering an area of 1km². In this dissertation, an analysis of data taken in 2007 with 26 IceTop stations operational at that time is presented. Air showers were reconstructed in order to determine the location of the shower core, their direction and the shower size. The relation between primary energy and shower size was determined from Monte Carlo simulations. An assumption was made about the chemical composition of cosmic rays. The information obtained in these simulations were then used to unfold the spectrum of measured shower sizes in order to obtain the all-particle cosmic ray energy spectrum. This was done independently for particles from three different zenith angle intervals. The result of the unfolding depends on the assumed primary composition. Due to the isotropy of cosmic rays, results obtained in different zenith angle intervals must agree. It has already been shown that this requirement can be used to constrain the range of possible assumptions on the chemical composition of primary particles. Good agreement of spectra from different zenith angle ranges has been found under the assumption of pure proton primaries, as well as for a mixture of protons and iron with a relatively large proton contribution at low energies and proton dominance at high energies. Under these assumptions the knee of the cosmic ray energy spectrum has been observed at energies between 3.97 and 4.20PeV. The spectral index below the knee is about -2.7 and varies between -3.08 and -3.15 above the knee. Pure iron as primary particles can be excluded at a high confidence level below 25PeV. Independent of the primary composition assumption a flattening of the energy spectrum with an index of about -3.0 has been observed above 30PeV.