Searches for supersymmetric partners of third generation quarks and measurement of the $t\bar{t} + Z$ 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 $\sqrt{s} = 7$ TeV and 8 TeV respectively. The focus of the work presented in this thesis is separated into two areas, measurement of the $t\bar{t}+Z$ process and searches for the supersymmetric partners to third generation quarks. Firstly, a search for $t\bar{t}+Z$ production using 4.7 fb$^{-1}$ of $\sqrt{s} = 7$ 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 $t\bar{t}+Z$ 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 $\tilde{b}_1 \rightarrow b \tilde{\chi}^0_1$ decay channel using 2.05 fb$^{-1}$ of $\sqrt{s}=7$ 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 $m_{\tilde{b}_1}-m_{\tilde{\chi}^0_1}$ plane. For a massless neutralino sbottom masses are excluded up to 390 GeV. For neutralino mass of 120 GeV sbottom masses are excluded for $275 < m_{\tilde{b}_1} < 350$ GeV. Finally two searches for stop squark pair production in the $\tilde{t}_1 \rightarrow t \tilde{\chi}^0_1$ decay channel are described, one using 4.7 fb$^{-1}$ of $\sqrt{s}=7$ TeV data and the other using 20.5 fb$^{-1}$ of $\sqrt{s}=8$ 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 $m_{\tilde{b}_1}-m_{\tilde{\chi}^0_1}$ 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.