Energy Reconstruction in the Hadronic Tile Calorimeter and Early SUSY Fully Hadronic Searches in ATLAS

The work presented in this thesis is framed in the pre-operation phase of the ATLAS experiment at LHC. Its first part is devoted to the energy reconstruction in the ATLAS Tile Calorimeter. In particular, the Optimal Filtering energy reconstruction algorithm implementation for offline uses and its validation studies with Monte Carlo and real data are presented. This algorithm combines a simple formulation with robustness in the reconstruction of signal amplitude and timing for data affected by electronics noise. Furthermore, the performance under a minimum bias pileup environment and the impact of this kind of physics noise are also shown. The second part of the thesis contains a contribution to the ATLAS Jet/Missing Transverse Energy (MET) and SUSY groups. Cleaning methods for events with large fake MET are proposed. These methods are based on one hand in the jet energy deposition in selected parts of the calorimeter system which are related with jet leakage or dead material effects and, on the other hand, in the use of the MET obtained just from the inner detector tracks. The application of these methods within SUSY analysis is also presented, discussing its QCD background reduction capabilities. Finally, the fully hadronic SUSY discovery analysis using Monte Carlo data with a signature characterized by at least 4 jets and no lepton in the final state. The cleaning methods described above are also included in the event selecti on and the discovery significance which can be achieved with this analysis is discussed for several points in the minimal SuperGravity parameter space.