Search for top-antitop quark resonances with the ATLAS detector at the Large Hadron Collider

The intriguing nature of the top quark, by far the heaviest particle in the Stan- dard Model of particle physics, has motivated the development of many theoretical extensions predicting the existence of new massive particles decaying to a pair of top-antitop quarks. The production of these hypothetical particles in proton-proton collisions at the Large Hadron Collider would reveal itself as a resonance in the expected smooth distribution of the top-antitop quark invariant mass. This thesis presents a search for such a new heavy particle decaying to a pair of top-antitop quarks in the semi-leptonic nal state. The analyzed data sample amounts to a total of 4.6 fb−1 at a proton-proton collision center-of-mass energy of 7 TeV. Novel techniques speci cally tailored to the identi cation of the decay products of highly energetic top quarks are developed and used. No evidence for resonant production of pairs of top-antitop quarks is found and, as a result, constraints are set on two theoretical models. Upper limits on the production cross-section times branching ratio are established at a 95% credibility level for a leptophobic Z′ boson from the Topcolor model, and a Kaluza-Klein gluon from the Randall-Sundrum model. The Z′ boson and the Kaluza-Klein gluon are excluded to exist (at a 95% credibility level) in the mass ranges 0.8-1.65 TeV and 0.8-1.88 TeV, respectively. The constraints de- rived in this thesis on the two theoretical models are more stringent than the ones obtained at other experiments, thanks to the large center-of-mass energy and the dedicated high-energy top quark identi cation techniques used.