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Lambacher, Marion (2007): Study of fully hadronic ttbar decays and their separation from QCD multijet background events in the first year of the ATLAS experiment. Dissertation, LMU München: Fakultät für Physik



The LHC is a top quark factory producing \ttbar events at a cross section of $833~\mathrm{pb}$ in NLO. This corresponds to about $8\cdot10^{6}$ \ttbar events in the first nominal year of the LHC at the initial low integrated luminosity of $10~\mathrm{fb^{-1}}$ being delivered. Approximately $44~\%$ of the \ttbar pairs decay hadronically into six jets. QCD multijet events with four to six final state partons are the main background to these \ttbar events with a cross section many orders of magnitude above the \ttbar multijet cross section. This study deals with the generation of fully hadronic \ttbar events and QCD multijet events with up to six final state partons and their measurement in the ATLAS detector via fast parameterized simulation. The characteristics of the QCD events with respect to the \ttbar signals are discussed and a cut-based selection for the separation of the \ttbar events from the QCD background is introduced. The presented analysis is designed to use the physical and technical information available in the start-up period of the LHC. The extraction of the \ttbar events results in more than 3000 remaining fully hadronic \ttbar events which can be separated from the QCD multijet background in the first year of the LHC. This analysis also includes the reconstruction of the top-mass peak from fully hadronic \ttbar events in the start-up period of the LHC and gives an estimate of the relative statistical uncertainty for the determination of the \ttbar production cross section of approximately $4~\%$ at an integrated luminosity of $10~\mathrm{fb^{-1}}$.