The production of a light Standard Model Higgs boson in association with a top-quark pair at the LHC is studied in a simulation of the ATLAS detector. The Higgs boson is assumed to decay into bb, and the top-quark pair to decay into lνb jjb. Tagging of all four b-jets and the full reconstruction of the final state are necessary to minimize the combinatorial background from assigning two jets to the decay of the Higgs boson and to discriminate the signal process from the large background from top-quark pairs with additional jets. New methods based on likelihood techniques are investigated in order to improve the reconstruction and the signal-background separation. They lead to substantial enhancement of the expected sensitivity of ATLAS to the ttH⁰ channel. Both a fast and the full simulation of the ATLAS detector are used, and a focus is set on the study of the ATLAS b-tagging performance and its parameterization in the fast simulation of the detector. <br /> Further, topological searches for neutral Higgs bosons h⁰ produced in e⁺e^- collisions at LEP in the process e⁺e^- to h⁰Z⁰ are investigated using data collected at center-of-mass energies of 183-209 GeV with the OPAL detector. These searches are based on studies of the recoil mass spectrum of Z⁰ to e⁺e^- and μ⁺μ^- events. They cover arbitrary decays of the h⁰ as well as the possibility that it might be stable. No indication for a signal is found in the data and upper limits on the cross section of h⁰Z⁰ production are calculated. The results can be interpreted in general scenarios independently of the decay modes of the h⁰. The examples considered are the production of a single new scalar particle with a decay width smaller than the detector mass resolution, and, for the first time, two scenarios with continuous mass distributions, due to a single very broad state or several states close in mass....

The ATLAS experiment at the future Large Hadron Collider at CERN uses a silicon pixel detector as the innermost tracking device. Measurements characterising the prototype ATLAS pixel readout chip FE-A/C are described and ...

Der Entwurf und die Charakterisierung des Auslesechips FED für den ATLAS Pixeldetektor in der strahlenharten DMILL Technologie werden in dieser Arbeit beschrieben. Bei einer Ereignisrate von 40 MHz und einer totalen ...

Three studies of different components of the object reconstruction with the ATLAS experiment using simulated data are presented. In each study, a method for the improvement of the reconstruction is developed and the ...

The measurement of the effective weak mixing angle with the ATLAS experiment at the LHC is presented. It is extracted from the forward--backward asymmetry in the polar angle distribution of the muons originating from $Z$ ...

This dissertation presents a search for long-lived, multi-charged particles using the ATLAS detector at the LHC. Motivation for this search arose from an unexploited search regime at ATLAS of stable massive particles with ...

In this thesis, a search for the H→b¯b decay mode of the Standard Model Higgs boson in associated production with a vector boson is presented. The search is based on the entire data-set recorded by the ATLAS experiment in ...

Physics studies of processes with t leptons in the final state, while challenging at hadron colliders, are of great importance at the LHC. The τ leptons provide important signatures in searches for the Higgs boson as well as for new physics in a wide range of theoretical models. Decays of Standard Model particles to τ leptons, in particular Z→ττ and W → τν_τ, are important background processes in those searches and their cross sections need to be measured first. This thesis reports the first observation of W → τν_τ decays and of hadronically decaying τ leptons with the ATLAS experiment at the LHC. The analysis is based on a data sample corresponding to an integrated luminosity of 546 nb^-1, which was recorded at a proton-proton centre-of-mass energy of 7TeV. A total of 78 data events are selected, with an estimated background of 11.1±2.3_(stat.) ±3.2_(syst.) events from QCD processes, and of 11.8±0.4_(stat.) ±3.7_(syst.) events from other W and Z decays. The observed excess of data events over the total background is compatible with the SM expectation for W → τν_τ decays, both in the number of events and in the shapes of distributions of characteristic variables....

The cross section of the Standard Model electroweak production of a single top quark in the t-channel has been measured using the LHC proton-proton collision data at $sqrt{s}=7$ TeV, $35 {\rm pb}^{-1}$ of integrated ...

Discovery prospects for Supersymmetry in final states with at least two tau leptons and large missing transverse energy are evaluated for LHC proton-proton collisions measured with the ATLAS detector. In a first study based on Monte Carlo simulation, a Supersymmetry signal selection is developed for fully hadronic, semileptonic and fully leptonic di-tau decays in events produced in proton-proton collisions with a center-of-mass energy of √s=10 TeV. The prospects of measuring the kinematic endpoint of the di-tau invariant mass distribution assuming an integrated luminosity of ∫ L dt=1 fb⁻¹ are evaluated for different mSUGRA scenarios along with the discovery reach. In a second study, a search for Supersymmetry in the fully hadronic channel is performed, using ∫ L dt=35 pb⁻¹ of ATLAS data collected with proton-proton collisions at √s=7 TeV between June 24^th and October 29^th 2010. Two events are observed, which is consistent with the Standard Model background expectation of N_SM = 0.77 ± 0.19^(stat)_-0.33^{+0.55 (syst)}. Interpreted in mSUGRA, limits on m₀ and m_1/2 are set for tan ß=40, a positive higgsino mixing parameter and A₀ =0 GeV or A₀ =-500 GeV. These limits constitute the first results of SUSY searches with tau leptons within the ATLAS experiment....
Das Entdeckungspotential von Supersymmetrie mit Endzust¨anden aus mindesten zwei Tau Leptonen und fehlender Transversalenergie in Proton-Proton Kollisionen wird untersucht, sowie die M¨oglichkeit einer Messung des Endpunktes der invarianten Masse der beiden Tau-Leptonen evaluiert. Hierf¨ur werden am LHC erzeugte und mit dem ATLAS-Detektor gemessene Ereignisse verwendet. Eine erste Studie basierend auf Monte Carlo Simulation entwickelt eine Supersymmetrie- Ereignisselektion f¨ur voll hadronische, semileptonische und voll leptonische Zerf¨alle der Tau-Paare f¨ur Ereignisse, die in Proton-Kollisionen mit einer Schwerpunktsenergie von √s=10 TeV produziert werden. Die Aussicht auf eine erfolgreiche Messung des kinematischen Endpunktes des Spektrums der invarianten Masse der Tau-Paare mit einer integrierten Luminosit ¨at von ∫ L dt=1 fb⁻¹ wird in verschiedenen mSUGRA Modellen untersucht und das Entdeckungspotential bestimmt. In einer zweiten Studie werden ∫ L dt=35 pb⁻¹ an ATLAS-Daten ausgewertet, die in Proton-Kollisionen mit √s=7 TeV zwischen dem 24. Juni und dem 29. Oktober 2010 aufgenommen wurden, und nach Hinweisen f¨ur Supersymmetrie im voll hadronischen Kanal gesucht. Die beobachtete Anzahl selektierter Ereignisse ist mit 2 Ereignissen in guter ¨ Ubereinstimmung mit der Standardmodellerwartung von N_SM = 0.77 ± 0.19^(stat)_-0.33^{+0.55 (syst)} Ereignissen. Dieses Ergebnis wird im mSUGRA Modell interpretiert, und es werden Grenzen f¨ur m₀ und m_1/2 im mSUGRA-Parameterraum mit tan ß =40, einem positiven Higgsino- Mischungsparameter und A₀ =0 GeV oder A₀ =-500 GeV gesetzt. Dies stellt das erste Ergebnis der ATLAS SUSY Suche mit Tau Leptonen dar....