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....

The top quark is the heaviest of the known fundamental particles in the Standard Model (SM) with a mass of 173.3±1.1GeV. It is the sixth quark in the SM and the weak isospin partner of the bottom quark. Properties such as the mass and the cross section have been measured since its discovery in 1995 with great precision at the proton-antiproton Fermilab Tevatron Collider, Chicago at a centre-of-mass energy of 1.8GeV and 1.96GeV. With the start of the proton-proton CERN Large Hadron Collider (LHC), Geneva, a new record in the centre-of-mass energy was achieved. The LHC opens a new energy frontier and probes the SM at energies of the electroweak symmetry-breaking scale and beyond. The measurement of the top-quark pair cross-section at this new energy is an important step, since it will either re-establish the SM at these energies by confirming the theoretical calculations or it can point to new phenomena that help to understand the so far not explained experimental findings in particle physics and cosmology.<br /> This thesis presents a study of the production cross-section measurement of topquark pair in the dilepton channel at the LHC with the ATLAS experiment at a centre-of-mass energy of √s =10TeV and for a data amount of L_int.=200 pb^-1. This study investigates the performance of the cross-section measurement with a cut-and-count analysis on Monte Carlo simulated samples. Important background processes are taken into account such as leptonically decaying Z-boson and single top-quark events. Data-driven background methods for Z-boson and fake event rates are discussed. A full list of systematic uncertainties is investigated and finally combined with the statistical uncertainties in a likelihood to extract the combined uncertainty. A “top mass”-like variable is presented that can help to distinguish dileptonic top-quark pair decays from similar signatures in new physics models. The combined sensitivity of the analysis in all dilepton decay-channels is a relative uncertainty on the cross-section measurement of:<br /> +3.1% -3.1%(stat) +9.6% -8.7% (syst) +26.2% -17.4% (lumi)<br /> Finally the study is compared to the same analyses performed at different centre-of-mass energies of 14 GeV and 7 GeV and two dilepton ttbar-pair event candidates in 280 pb^-1 of first ATLAS data are presented....

The production of a single top quark in association with a W boson (Wt) is measured with the ATLAS detector using proton-proton collision events with one lepton, three jets and missing transverse energy at √s = 8 TeV. ...

A measurement of the top-quark mass and the t<span style="text-decoration: overline">t</span>Z cross section is performed using 4.6 fb^-1 and 20.3 fb^-1 of data from proton-proton collisions collected ...