Optimisation of the event selection for the single top quark production in association with a Z boson at the ATLAS detector

The unprecedented amount of data collected by the ATLAS Experiment at the Large Hadron Collider at CERN has made it possible to measure rare processes, foreseen by the Standard Model (SM), that have never been observed before. The study of the top quark is of primary importance for the ATLAS physics program. Being the heaviest particle of the SM, with a coupling to the Higgs boson close to one, the top quark can help shedding light over the electroweak symmetry breaking mechanism. The production of a single top quark in association with a Z boson ($tZq$) has recently been observed by the ATLAS Experiment. The cross section of this process, predicted by the SM to be 102 fb, has been measured with an uncertainty of 15%. In this thesis, the proton-proton collision data collected by the ATLAS Experiment during Run2 will be analysed to improve the measurement of the $tZq$ cross-section. In particular, the aim is to refine the selection of the events to enhance the signal acceptance and to improve the rejection of the backgrounds by using the most recent object reconstruction algorithms available. In the first two chapters I am going to give a brief description of the Standard Model and the top quark physics. In the third chapter I will describe how the ATLAS Experiment works. In the fourth chapter I will present my optimisation studies and their results.