Search for New Physics in events with 4 top quarks in the ATLAS detector at the LHC

This thesis presents the search for New Physics in events with four top quarks using the data collected in proton-proton collisions by the ATLAS experiment at the LHC. The dataset corresponds to the one taken during all 2011 at 7 TeV and a part of 2012 at 8 TeV. The analysis focuses on a final state with two leptons (electrons and muons) with the same electric charge. This signature is experimentally favored since the presence of two same-sign leptons in the final state allows to reduce the background coming from Standard Model (SM) processes. The results are interpreted in the context of a low energy effective field theory, which assumes that New Physics at low energy can manifest itself as a four right-handed top contact interaction. In this context, this analysis allows testing a class of beyond-the-SM (BSM) theories which at low energy can manifest in this way. Backgrounds to this search have been estimated using simulated samples and data-driven techniques. Different sources of systematic uncertainties have been also considered. The final selection of events has been optimized by aiming at minimizing the expected upper limit on the four tops production cross-section in case of no signal events found. The signal region is then analyzed by looking for an excess of events with respect to the predicted background. No excess of events has been observed, and the observed upper limit on the four tops production cross-section has been computed. This limit is then translated to an upper limit on the coupling strength of the model. An upper limit on the four tops production cross-section in the SM has been also computed in the analysis performed at 8 TeV. In addition to the physics analysis of the four tops signal, some studies about the LASER calibration system of the ATLAS Tile calorimeter are presented. In particular, they are related to the photodiodes system used to measure the intensity of the laser light in the LASER system.