Dr Kirika Uchida (Universität Bonn)
Many physics analyses with the ATLAS detector expect to have jets originating from b-quarks. Algorithms that allow to identify those jets are thus of great importance and it is crucial to understand their performance, namely the b-tagging efficiencies, misidentification efficiencies and calibration. The final state of single lepton top pair decays is characterized by at least four jets among which at least two are b-jets. This b-enriched sample provides a perfect environment for calibration of b-tagging algorithms for analyses with large multiplicity of high transverse momentum jets, for example Higgs and SUSY searches. On the other hand, the dilepton channel guarantees a high purity of selected ttbar events and ensures a high reliability of the real b-tagging efficiency estimation. This approach takes advantage of the large cross-section of the top quark pair production at the LHC and a good understanding of this process after the initial phase of data taking with the ATLAS detector. To measure the b-tagging efficiency in the single lepton channel a slightly modified tag and probe method is applied to ttbar events selected from data. Alternatively, in both single lepton and dilepton channels, b-tagged jets in an event can be counted. This method provides as well a ttbar production cross-section estimation. Thanks to the large amount of data collected by ATLAS in 2011 those studies are not much affected by statistical uncertainty and an accurate measurement of the efficiencies in many jet transverse momentum bins is possible.
Agnieszka Leyko Leyko (University of Bonn)