Measurement of the associated production of a vector boson (W, Z) and top quark pair in the opposite sign dilepton channel with pp collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector

The heaviest known elementary particle predicted by the Standard Model (SM), the top quark, was discovered in 1995 by the CDF and D0 collaborations. The large top quark mass translates into a coupling to the Higgs boson close to unity and, therefore, it is expected that the top quark may play a special role in electroweak symmetry breaking. Among its various properties, the study of the coupling between the top quark and the Z-boson would allow to test the SM prediction, and search for any deviations caused by possible new physics signals. The first step towards the measurement of such a coupling at hadron colliders, is the observation of the associated production of a Z-boson and a top quark pair, $t\bar{t}Z$. The measurement of the $t\bar{t}Z$ production cross section, together with that of the associated production of a W-boson and a top quark pair, jointly denoted as $t\bar{t}V$, using the data collected by the ATLAS experiment at a centre-of-mass energy of $\sqrt{s} = 8$ TeV in final states with two leptons of opposite sign charge, is the main topic of this thesis. The dataset corresponds to an integrated luminosity of 20.3 fb$^{-1}$. The implementation of advanced multivariate techniques, such as a neural networks, together with a careful design of the fit regions and the usage of a profile likelihood fit to extract the signal cross section and reduce the uncertainties, are used to increase the sensitivity of the analysis. The combined cross section results, with final states of two or three charged leptons, are obtained in various scenarios. Assuming $t\bar{t}W$ production at the Standard Model rate, predicted by next-to-leading-order QCD calculations, the ratio of the measured $t\bar{t}Z$ signal strength to the Standard Model expectation is found to be $\mu t\bar{t}W$ = 0.73 +0.29 -0.26, corresponding to a 3.2 sigma excess over the background-only hypothesis. Assuming $t\bar{t}Z$ production at the Standard Model rate, the $t\bar{t}W$ signal strength is found to be $\mu t\bar{t}W$ = 1.25 +0.57 -0.48, corresponding to a 3.1 sigma excess over the background-only hypothesis. The combined $t\bar{t}Z$ and $t\bar{t}W$ signal strength is found to be $\mu t\bar{t}V$ = 0.89 +0.23 -0.22, corresponding to a 4.9 sigma excess over the background-only hypothesis. A simultaneous measurement of the $t\bar{t}Z$ and $t\bar{t}W$ signal strengths yields $\mu t\bar{t}Z$ = 0.71 +0.28 -0.26 and $\mu t\bar{t}W$ = 1.30 +0.59 -0.48, corresponding to a 3.1 sigma excess over the background-only hypothesis for both $t\bar{t}Z$ and $t\bar{t}W$ processes. Evidence of both $t\bar{t}Z$ and $t\bar{t}W$ processes is obtained. All measurements are consistent with next-to-leading-order theoretical calculations for $t\bar{t}Z$ and $t\bar{t}W$ processes.