Search for $tZ$ production via Flavour Changing Neutral Currents with the ATLAS experiment at the LHC

The subject of the present dissertation is the search for $tZ$ event production by Flavour Changing Neutral Currents (FCNC) through the analysis of data collected by the ATLAS detector of the Large Hadron Collider located in CERN. The FCNC processes are very rare in the Standard Model of Particle Physics since they are not allowed at tree-level and extremely suppressed at loop-level. However, this processes have a higher probability to occur in several models beyond the Standard Model. In this thesis, a search for the production of a top quark and a boson $Z$ via FCNC in a trileptonic topology is discussed. The final state of the $tZ$ production consists in a pair of leptons with opposite sign and same flavour having a mass compatible with the decay of a $Z$ boson, a charged lepton and a neutral lepton classified as coming from the decay of a top quark and finally a jet coming from a bottom quark ($b$-tagged jet). The analyzed data coming from proton-proton collisions at the centre-of-mass energy of $\sqrt{s}$ = 13 TeV was collected in the data-taking period between July and November of 2015 corresponding to an integrated luminosity of 3.21 fb$^{-1}$. A complementary study of possible discriminant variables is presented. Through this study, expected upper limits at $95\%$ confidence level on the cross-section of $tZ$ production via FCNC were obtained. An interpretation of these limits in terms of branching ratios for $t$ $\rightarrow$ $qZ$ and the $tZq$ coupling is also presented.