Search for a Z', a New Heavy Gauge Boson Beyond the Standard Model, as a Resonance Within a tt ̅tt ̅ Event in a Fully-Hadronic Decay Channel, With the ATLAS Detector

The Standard Model of particle physics is the ultimate cumulation of many theories and experimental results aiming to describe the elementary particles constructing the universe around us at its most fundamental level, and to define the interactions perceived by these tiny building blocks. Despite being one of the greatest achievements in modern physics, the SM, like any other model, has some limitations and shortcomings, for this reason, many approaches beyond the standard model (BSM) attempted to overcome these flaws. All these BSM approaches suggest new particles outside the contents of the SM, and the discovery of these particles is mandatory for their approval. In this Master thesis, we were interested in probing the observation of a new heavy gauge boson outside of the SM framework, as a resonance state within a very rare event predicted by the SM, the four top-quarks tt ̅tt ̅, using the ATLAS detector. Such a boson, called Z', is predicted by several BSM propositions, namely the Sequential Standard Model (SSM), the Left-Right Symmetric Model (LRM), and the Grand Unified Theories (GUTs). After introducing the SM, its limitations, and the requirement for BSM representations, we exposed different production and decay modes of the top quark within and beyond the SM. Next, we presented the experimental setup, specifically, the Large Hadron Collider (LHC), its acceleration chain, and the ATLAS detector. The process of translating the electrical signals from the detector to identified objects and reconstructed paths is mentioned for the physic objects used in this work, i.e. electrons, muons, and jets. Finally, we have proceeded with a fully hadronic event selection, using top-xaod, the main AnalysisTop program. Accomplishing a full and complete analysis was further on than this manuscript's scope, taking into consideration the limited abilities of a master’s student, and the research tight timeframe. However, in addition to the bibliography mentioned above, there were two main tasks addressed throughout this study journey. On the theoretical side, we re-calculated the BRs of the 12 deferent decay channels of a tt ̅tt ̅ event, and on the analysis side, we adapted the configuration file used as input for the main program in AnalysisTop, the top-xaod, to suit a fully-hadronic decay channel. The former task revealed results tunning to the ones we received, moreover, the sum of BRs we have calculated was 1.0001, while it was 0.996 in the given document, and this is due to us considering two digits after decimal instead of just one taken in the same document. As for the latter task, the configuration file was tested for different MC samples; MC16a, MC16d, MC16f, and MC16e, as well as data samples, and it did run with only some warnings but without any errors. However, we only presented in the last section of the last chapter some results from the output file we have obtained using the MC16e sample, and this is because it was the one with the greatest number of entries, so the plots are quite satisfying.