Die Physik des Top-Quarks ist eine Schlüsselkomponente im Forschungsprogramm des ATLAS-Experiments am CERN. In dieser Arbeit werden Untersuchungen zur Leistungfähigkeit von Jet-Triggern für Top-Quark-Ereignisse präsentiert und zwei datenbasierte Methoden zur Abschätzung der Multijet-Triggereffizienz und des W+Jets-Untergrundes in Top-Quark-Ereignissen in ATLAS eingeführt. In einer tag-and-probe Methode, basierend auf einer einfachen und allgemeinen Ereignisselektion und einem hochenergetischen Lepton als Tag, wird die Möglichkeit zur Bestimmung der Multijet-Triggereffizienz aus Daten heraus evaluiert, und es wird gezeigt, dass die Methode in der Lage ist, die Effizienz ohne signifikante Verfälschung durch die Tag-Selektion zu bestimmen. In der zweiten datenbasierten Analyse wird eine neue Methode zur Abschätzung des W+Jets-Untergrundes in ATLAS eingeführt. Durch die Definition von signal- und untergrunddominierten Bereichen in Jet-Muliplizität und Pseudorapidität des Leptons wird der Anteil der W+Jets-Ereignisse aus der untergrunddominierten in die signaldominierte Region extrapoliert. Es wird gezeigt, dass die Methode, mit einer integrierten Luminosität von 100 pb^−1 bei sqrt(s) = 10 TeV, in der Lage ist den Untergrundbeitrag als Funktion der Jet-Muliplizität mit etwa 25% Genauigkeit im Großteil der signaldominierten Region zu bestimmen. Diese Arbeit umfaßt zudem eine Studie zum thermischen Verhalten und der erwarteten thermischen Leistung des Pixel-Detektors in ATLAS. Alle Messungen, durchgeführt während der Inbetriebnahme des Systems in 2008/09, zeigen Ergebnisse innerhalb der Spezifikationen beziehungweise deuten auf deren Einhaltung auch nach mehreren Betriebsjahren unter LHC-Bedingungen hin. / The physics of the top quark is one of the key components in the physics programme of the ATLAS experiment at the Large Hadron Collider at CERN. In this thesis, general studies of the jet trigger performance for top quark events using fully simulated Monte Carlo samples are presented and two data-driven techniques to estimate the multi-jet trigger efficiency and the W+Jets background in top pair events are introduced to the ATLAS experiment. In a tag-and-probe based method, using a simple and common event selection and a high transverse momentum lepton as tag object, the possibility to estimate the multijet trigger efficiency from data in ATLAS is investigated and it is shown that the method is capable of estimating the efficiency without introducing any significant bias by the given tag selection. In the second data-driven analysis a new method to estimate the W+Jets background in a top-pair event selection is introduced to ATLAS. By defining signal and background dominated regions by means of the jet multiplicity and the pseudo-rapidity distribution of the lepton in the event, the W+Jets contribution is extrapolated from the background dominated into the signal dominated region. The method is found to estimate the given background contribution as a function of the jet multiplicity with an accuracy of about 25% for most of the top dominated region with an integrated luminosity of above 100 pb^−1 at sqrt(s) = 10 TeV. This thesis also covers a study summarising the thermal behaviour and expected performance of the Pixel Detector of ATLAS. All measurements performed during the commissioning phase of 2008/09 yield results within the specification of the system and the performance is expected to stay within those even after several years of running under LHC conditions.

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This thesis presents the design qualification and construction of the Beam Scintillator Counter (BSC) for the CMS Collaboration at CERN in 2007 - 2008. The BSC detector is designed to aid in the commissioning of the Compact Muon Solenoid (CMS) during the first 2 years of operation and provide technical triggering for beam halo and minimum-bias events. Using plastic scintillator tiles mounted at both ends of CMS, it will detect minimum ionizing particles through the low-to-mid luminosity phases of the Large Hadron Collider (LHC) commissioning. During these early phases, the BSC will provide probably the most interesting and widely used data of any of the CMS sub-detectors and will be employed in the track based alignment procedure of the central tracker and commissioning of the Forward Hadron Calorimeter.

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Searches for the supersymmetric partner of the top quark (stop) are motivated by natural supersymmetry, where the stop has to be light to cancel the large radiative corrections to the Higgs boson mass. This thesis presents three different searches for the stop at √s = 8 TeV and √s = 13 TeV using data from the ATLAS experiment at CERN’s Large Hadron Collider. The thesis also includes a study of the primary vertex reconstruction performance in data and simulation at √s = 7 TeV using tt and Z events. All stop searches presented are carried out in final states with a single lepton, four or more jets and large missing transverse energy. A search for direct stop pair production is conducted with 20.3 fb−1 of data at a center-of-mass energy of √s = 8 TeV. Several stop decay scenarios are considered, including those to a top quark and the lightest neutralino and to a bottom quark and the lightest chargino. The sensitivity of the analysis is also studied in the context of various phenomenological MSSM models in which more complex decay scenarios can be present. Two different analyses are carried out at √s = 13 TeV. The first one is a search for both gluino-mediated and direct stop pair production with 3.2 fb−1 of data while the second one is a search for direct stop pair production with 13.2 fb−1 of data in the decay scenario to a bottom quark and the lightest chargino. The results of the analyses show no significant excess over the Standard Model predictions in the observed data. Consequently, exclusion limits are set at 95% CL on the masses of the stop and the lightest neutralino.