Search for flavour-changing neutral-current top quark decays to c-quark and Z boson using the Atlas detector at the LHC

The main focus of this thesis is the search for the t → Zc process in the proton–proton collisions data collected by the ATLAS detector at the Large Hadron Collider located at CERN. The flavour-changing neutral-current (FCNC) processes are forbidden at tree-level and highly suppressed at loop-level which is why they are very rare phenomena in the Standard Model of particle physics. However, this processes have a higher probability to occur in several theories beyond the Standard Model where the suppression could be relaxed and the loop diagrams mediated by new bosons could contribute. The FCNC top-quark decays t → Zc are searched in tt pair events with one top quark decaying through the t → Zc channel and the other through the dominant Standard Model mode t → Wb. The analysed data were recorded at a center-of-mass energy of 13 TeV and correspond to the full Run-2 dataset with an integrated luminosity of 139 fb−1 . The data are consistent with Standard Model background contributions, and, at 95% confidence level the search sets observed (expected) upper limits of 11.8 × 10−5 (9.5 × 10−5 ) on the t → cZ branching ratio, constituting the most stringent limit to date and improving the previous ATLAS results by a factor of 2 (2.5).