Semi- and Dileptonic Top Pair Decays at the ATLAS Experiment

The Large Hadron Collider, starting in 2008, will be a "top
factory" as top-antitop pairs will be produced with a cross section
of about 830 pb at an instantaneous luminosity of 10^33 cm^-2 s^-1
during the first year. With about 30% probability top pairs decay
semileptonically into a final state with four jets, lepton
(electron or muon) and respective neutrino. For another 5% of the
top pair events a dileptonic decay is expected. Here the final
state signature is composed of two jets, two leptons and two
neutrinos. In this thesis the precision for a top pair cross
section measurement at the ATLAS experiment in the semileptonic and
dileptonic channels with cut based analyses, applicable to the
first data, was estimated. The analysis of the semileptonic decay
focused especially on the study of background from QCD events
either with leptons from semileptonic hadron decays or from hadrons
falsely identified as electrons by the calorimeter. For the first
10 fb^-1 and assuming a fake electron probability of 10^-3 a
precision for the cross section times the branching ratio of +-
0.5(stat) +- 30.4(syst) +- 24.0(lumi) pb has been estimated,
corresponding to a relative precision of 16% for the theoretically
predicted cross section times branching ratio of about 240 pb. The
analysis in the dileptonic channel achieves a precision of +-
0.2(stat) +- 2.5(syst) +- 2.6(lumi) pb which translates into a
relative error of 10% for the cross section times branching ratio
of around 38 pb. The errors for both the semileptonic and the
dileptonic channel are expected to improve as progress is made on
the luminosity determination and the knowledge of the backgrounds
from comparisons with measured data. A measurement of the
cross-section ratio between the dileptonic and semileptonic channel
is sensitive to scenarios of new phenomena with competitive top
quark decay modes such as decays involving a charged Higgs boson.
It has been estimated that such a ratio should be measurable with a
relative precision of +- 0.7%(stat) +- 7.7%(sys) +- 3.1%(lumi)
during the first year of ATLAS data-taking. Even though the
systematic errors partially cancel in such a ratio the total
uncertainty is still around 8% as the background estimates rely on
theoretical predictions. This should also improve as soon as the
models can be tested against measured data.