Searches for supersymmetric partners of third generation quarks and measurement of the ttbar+Z process using the ATLAS detector at the CERN Large Hadron Collider

This thesis presents results using data collected by the ATLAS experiment during 2011 and 2012 at √s = 7 TeV and 8 TeV respectively. The focus of the work presented in this thesis is separated into two areas, measurement of the tt ̄+Z process and searches for the supersymmetric partners to third generation quarks.

Firstly, a search for tt ̄+ Z production using 4.7 fb−1 of √s = 7 TeV data is described including the generation of the dedicated simulated signal Monte Carlo samples for the analysis. The result is interpreted in terms of a 95% probability upper limit on the tt ̄+ Z production cross section of 0.71 pb. This is compatible with NLO Standard Model prediction of 0.14 pb.

Secondly, a number of searches for the supersymmetric partners of bottom
(sbottom) and top (stop) quarks are described. The first is a search for sbottom squark pair production in the b_1 → b_1 χ^0_1 decay channel using 2.05 fb−1 of √s = 7 TeV ATLAS data. No significant excess is observed above the Standard Model expectation and exclusion limits at the 95% confidence level are set in the m_b_1 − m_χ^0_1 plane. For a massless neutralino sbottom masses are excluded up to 390 GeV. For neutralino mass of 120 GeV sbottom masses are excluded for 275 < m_b_1 < 350 GeV. Finally two searches for stop squark pair production in the t_1 → t_! χ^0_1 decay channel are described, one using 4.7 fb−1 of √s = 7 TeV data and the other using 20.5 fb−1 of √s = 8 TeV data. Again, no significant excess is observed above the Standard Model expectation and exclusion limits at the 95% confidence level are set in the m_t1 − m_χ^0_1 plane. For a nearly massless neutralino, stop masses between 320 and 660 GeV are excluded. For neutralino mass of 150 GeV, stop masses are excluded between 400 and 620 GeV.