A measurement of the production cross-section for top quark pairs($\ttbar$) in pp collisions at $\sqrt{s}=7 \TeV$ is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron e or muon μ) with large missing transverse energy and at least four jets, and dilepton (ee, μμ or eμ) with large missing transverse energy and at least two jets. In a data sample of 2.9 pb-1, 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non-$\ttbar$ Standard Model processes are estimated using data-driven methods and determined to be 12.2±3.9 events and 2.5±0.6 events, respectively. The kinematic properties of the selected events are consistent with SM $\ttbar$ production. The inclusive top quark pair production cross-section is measured to be $\sigmattbar=145 \pm 31 ^{+42}_{-27}$ pb where the first uncertainty is statistical and the second systematic. The measurement agrees with perturbative QCD calculations.

Measurement of the top quark-pair production cross section with ATLAS in pp collisions at root s=7 TeV