Search for $t\bar{t}$ resonances in the lepton+jets channel in proton-proton collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector

The Standard Model of Particle Physics is a well proven model for the understanding of the constituents and interactions of ordinary matter. Remaining open questions hint at new physics beyond the Standard Model, motivating theories comprising extensions with new heavy particles decaying into top/anti-top pairs. The ATLAS experiment at the Large Hadron Collider at CERN recorded an integrated luminosity of 21.3 fb$^{-1}$ of data in 2012 at a center of mass energy of $\sqrt{s} = 8$ TeV, providing an increased sensitivity for heavy resonances with invariant masses in the TeV scale. This thesis presents an analysis of top/anti-top decays with the full 2012 dataset, reconstructing the full decay of the top/anti-top system in the lepton+jets channel with a signature of 4 jets, one charged lepton (electron or muon) and missing transverse energy. The decays are analyzed in resolved and boosted event topologies with different reconstruction and tagging techniques of the decay products. Special emphasis is set on the calculation of the dominant background from W+Jets processes and on improved top tagging techniques. The final discriminant is the invariant mass of the full top/anti-top system, which is used to set and/or improve the exclusion limits on specific heavy resonances models, since no significant deviations of the expectations are found.