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....

A multi-channel high-precision low-power time-to-digital converter application specific integrated circuit for high energy physics applications has been designed and implemented in a 130 nm CMOS process. To reach a target ...

<p>An evaluation of the discovery potential for NMSSM Higgs bosons of the ATLAS experiment at the LHC is presented. For this purpose, seven two-dimensional benchmark planes in the six-dimensional parameter space of the ...

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 ...

So far experimentally not ruled out, the stealthy Higgs scenario proposes a hidden scalar sector, to which only the Higgs has non-vanishing and possibly large couplings. Due to decays into the scalars the Higgs acquires a ...

The ATLAS experiment at the Large Hadron Collider (LHC) is measuring proton-proton collisions at a centre-of-mass energy of √s=7 TeV since 2010, an energy never reached before in collider experiments. A wide variety of ...

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 ...

Z boson production at the LHC presents a good opportunity to test Standard Model predictions. This thesis reports the measurement of the Z boson transverse momentum distribution in proton-proton collisions at sqrt(s)=7TeV, ...

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....