Measurement of the Top quark pair production cross section with ATLAS detector

This thesis describes two measurements of the $t\overline{t}$ production cross-section, the inclusive $\sigma_{t\bar{t}}$ and $\sigma_{t\bar{t}j}$ in association with at least one jet in lepton + jets channel with ATLAS detector. The measurements employ multivariate technique with a binned maximum likelihood template fit. Several kinematic variables of the reconstructed collision events are selected to discriminate $t\overline{t}$ signal events from the various background sources. The inclusive $\sigma_{t\bar{t}}$ measurement uses approximately 35.3pb$^{-1}$ of ATLAS data collected in 2010 and selecting events with one isolated high-$p_{T}$ electron or muon, large missing transverse energy and two jet bins of 3 or at least 4 high-$p_{T}$ jets. The measured $t\overline{t}$ cross section assuming the top quark mass of 172.5 GeV yields : \[ \sigma_{t\bar{t}}=173\pm17{\rm (stat.})_{-17}^{+20}{\rm (syst.)}\pm6{\rm (lumi.)\,pb} \] The measured value is in a good agreement with the Standard Model prediction The second measurement is performed on ATLAS data collected in 2011 corresponding to an integrated luminosity of 4.7 fb$^{-1}$. The same analysis technique and event selection employed as the first measurement but in 4th and 5th jet bins. The cross-section for $\sigma_{t\bar{t}}$ with at least one additional jet is found to be : \[ \sigma_{t\bar{t}j}=102\pm2{\rm (stat.)}\pm6{\rm (ISR/FSR)}_{-25}^{+22}{\rm (syst.)\,pb} \] Finally, the results of the study on PiN diodes radiation hardness are presented. Results were performed with several irradiation beams to investigate the proper optical components for ATLAS Pixel upgrade.