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

Abstract

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 $\int$Ldt = 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 $X$2 minimisation technique. Official Monte Carlo samples have been used to simulate the t¯t signal and background contributions in order to perform simulated experiments to obtain the statistical and systematic uncertainties. The results obtained from the measurement of the top parameter are: Г$_{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