Measuring the Top Quark Pair Production Cross Section in ATLAS

This thesis presents measurements of the top quark pair production cross section us- ing the ATLAS stream test data. The cross section is determined separately for the semileptonic mode, where events in the electron plus jets channel are isolated, and the dilepton mode, where events in the electron-muon channel are considered. The cross section is determined both with and without using tagging of b-quark jets. For the semileptonic mode, the measured cross section is σtt ̄ = 740 ± 57 pb without b-tagging and σtt ̄ = 788 ± 50 pb when using b-tagging. For the dilepton mode, the cross section is σtt ̄ = 655 ± 73 pb without b-tagging and σtt ̄ = 704 ± 86 pb when including b-tagging.