Search of new resonances decaying into top quark pairs with the ATLAS detector at the LHC and jet calibration studies

The Standard Model (SM) of particle physics, built over the last half century, provides a nearly complete picture of the known particles and the way they interact with each other. However, there are still some missing pieces in the SM of experimental and theoretical order. Several physical models, called Beyond-the-SM (BSM) theories, has been proposed in the last years to try to account for one or more of the SM's open questions. The validity of the SM and/or of the BSM theories can be tested at high energy experiments and is at this point that the Large Hadron Collider (LHC) experiment enters in the scene. The LHC is the largest and highestenergy particle accelerator ever built. Around 10000 physicists and engineers around the world are taking part in this experience by developing new techniques and approaches to identify the interesting physics buried in the complex environement produced in the LHC pp collisions. At the four collision points of the LHC, detectors have been placed to study the high-energy collisions. One of these detectors is ATLAS, a general purpose detector with an extensive initial physics program that includes precision measurements in the SM frame, the search of the Higgs boson and the search of signatures of new physics. An example of new physics signatures would be the existence of a new heavy particle that decays into top-quark pairs, a top pair resonance. Several BSM theories predicts this kind of heavy resonances that strongly couples to top quarks due to its high mass. This thesis presents the results of the search for new resonances that decay to top-quark pairs in the lepton plus jets nal state using the rst 2.05 fb􀀀1 of data collected in 2011 by the ATLAS detector. Related to this search, performance studies of the Jet Vertex Fraction (JVF) in top-quark pairs topologies are presented too. JVF is a variable that can be used to reduce the pile-up eects to improve the precision and sensitivity of physics analyses at high luminosities. The lepton plus jets nal state is constituted by six individually identied decay products: four jets, an electron or muon, and a neutrino. The understanding of the jet calibration has an important role in this analysis due to the presence of jets in the nal state. During the rst two years of data analysis in ATLAS the jet energy scale was obtained using a simple calibration approach, easy to understand and easy to derive systematic uncertainty for it, but with a low performance regarding the jet energy and angular resolution and the avor sensitivity. An extension of this simple calibration called Global Sequential (GS) calibration has been proposed in ATLAS. GS uses several jet properties to improve the jet energy resolution and reduce the jet response avour sensitivity. The performance, the data validation and the associated systematic uncertainty derivation of the GS scheme are also presented in this thesis.