Direct measurement of the top-quark width with the ATLAS experiment

In this thesis a measurement of the total decay width of the top quark, Г_{top} is presented using selected semi-leptonic {t¯t} event candidates, recorded from protonproton collisions at a centre-of-mass energy of \sqrt{s} = 8 TeV with the ATLAS experiment at CERN in Geneva, Switzerland. The total recorded integrated luminosity that was used for this measurement equals to \intLdt = 20.3 fb^{−1}. The analysis includes the two event channels: e + jets and μ + jets. The reconstruction of the {t¯t} system is achieved by using a per-event X2 minimisation technique. Official Monte Carlo samples have been used to simulate the t¯t signal and background contributions in order to perform simulated experiments to obtain the statistical and systematic uncertainties. The results obtained from the measurement of the top parameter are: Г_{top} = 1.65 GeV for the μ + jets channel and Г_{top} = 0.81 GeV for the e + jets channel. The upper limits obtained from the measured values and the analysis total uncertainties are: Г_{top} < 4.60 GeV at 68 % C.L. and Г_{top} < 7.16 GeV at 95 % C.L. for the μ + jets channel, and Г_{top} < 4.40 GeV at 68 % C.L. and Г_{top} < 7.51 GeV at 95 % C.L. for the e + jets channel. These results are consistent with the expected SM value Г_{top} ≈ 1.33 GeV.