The Large Hadron Collider (LHC) at the international particle physics laboratory CERN in
Switzerland is currently the most powerful particle accelerator on earth. This thesis presents
analyses of proton-proton collisions at the energy √s = 7 TeV, recorded by ATLAS, one of the
detectors at the LHC. The goal of the LHC and its detectors is to find new phenomena not described
by the Standard Model (SM) of particle physics.The top quark is the heaviest known elementary
particle and it is produced in very large numbers at the LHC. Measuring the production crosssection
of top pairs (ttbar) is important for many reasons: for validating the strong production
mechanism of the SM, for commissioning and calibration of the detector and analysis software
and because several scenarios for physics beyond the SM predict changes to the ttbar production
cross-section.Five different measurements of the ttbar cross-section will be presented in this
thesis. The first three are measurements of the total cross-section, the fourth is a simultaneous
measurement of the ttbar, Zττ and WW cross-sections and the fifth is a measurement of the
relative differential ttbar cross-section. The most accurate measurement of the total cross-section
is 176 pb with a total uncertainty of 9%, and the relative differential cross-section for ttbarmasses
above ~ 1 TeV is 0.007 1/TeV with an uncertainty of 43%. Both values agree with the SM
predictions.Measurements or searches in particle physics often have to be conducted in the presence
of uninteresting background processes. Reducing and providing estimates of these backgrounds
is one of the main analysis tasks. Many backgrounds can be simulated with sufficiently good
accuracy. However, the background due to mis-identified leptons cannot be accurately simulated.
This thesis presents and evaluates a method for estimating this background from data, and this is
then used in the total ttbar cross-section measurements.
