Study of fully hadronic ttbar decays and their separation from QCD multijet background events in the first year of the ATLAS experiment

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 $10~\mathrm{fb^{-1}}$.