Study of Standard Model processes with leptons of high transverse momentum with the ATLAS detector

This PhD thesis has taken up various aspects in experimental particle physics by analyzing the first data of the ATLAS detector. The main subject of the thesis is the production cross section measurement of the ZZ process at sqrt{s} = 7 TeV of proton-proton collisions. In order to study this, the author has contributed to various topics, such as detector related issues, the study of standard candle processes and Monte Carlo studies. In specic, this thesis presents a detailed study for the improvement of the hit position and peaking time resolution of the CSCs with 0.6% and 0.4% relative errors respectively. This thesis has also contributed to the first Z inclusive cross section measurement at sqrt{s} = 7 TeV, by analyzing the first 316 nb-1 of data recorded by the Atlas detector. The Z->ll cross section measurement is used as a standard candle for detector performance assessment as well as for the tuning of theoretical predictions at the new energy regime. The fiducial and total cross sections have been found to be: σ_fid(Z->ee) = 0.33 pm 0.04(stat) pm 0.03(syst) pm 0.04 (lumi) nb σ_tot = 0.75 pm 0.09 (stat) pm 0.08(syst) pm 0.08(lumi) nb and are in agreement with the Standard Model NNLO predictions of 0.96 pm 0.05 nb. This thesis studies the ZZ->4l process and completes the analysis with the first 1 fb-1. The fiducial and total cross section were determined to be: σ_fid(ZZ->llll) = 19.4^{+6.3}_{-5.2} (stat.) ^{+0.9}_{-0.7} (syst.) ^{+0.9}_{-0.6}(lumi) fb σ_tot = 8.5^{+2.7}_{-2.3} (stat) ^{+0.4}_{-0.3}(syst.) pm 0.3 (lumi) pb and are statistically consistent with the SM NLO total cross section of 6.5^{+0.3}_{-0.2} pb Finally, the current thesis searches for doubly charged Higgs bosons and the Randal-Sundrum Graviton in the same final states as in the ZZ->4l analysis. In the former, an upper limit on the fiducial cross section of four lepton production has been set at 4.7 fb and in the graviton case, an upper limit on the production cross section of ZZ pairs from high mass resonances was established, within the fiducial region, at 0.92 pb. The observed and expected 95% C.L. lower limit on the mass of the graviton is MG > 575 GeV.