Performances du calorimètre électromagnétique et recherche de nouveaux bosons de jauge dans le canal diélectron auprès du détecteur ATLAS

The Standard Model of particle physics has known a tremendous rise during the twentieth century. Built up from the early 1930s to the 1970s, this theory describing elementary particles and their interactions has now been intensively tested by LEP and Tevatron colliders. Besides its success, some questions remain and theories attempting to go beyond the Standard Model have emerged. Many of them are predicting the existence of a new gauge boson Z', which is supposed to be observable at the TeV scale. Data recorded by the LHC since autumn 2008 are a new opportunity to check the consistency of the Standard Model and to search for new physics evidence. Work that has been done by the ATLAS collaboration during the last four years has focused on understanding detector's behaviour and analysing the very first collected collisions. This thesis is reflecting these two aspects. The first part of this thesis describes the caracterisation of a pathology of the ATLAS liquid argon calorimeter electronics and the study of coherent noise bursts that have been observed since the beginning of ATLAS operation. The policy deployed to preserve data quality is also detailed. The second part of this book is focusing on the search for a new Z' gauge boson. If this particle was to exist, its decay into an electron and a positron would lead to a new massive resonance in the dielectron invariant mass spectrum. Therefore electron reconstruction and identification performance is closely looked at, especially at high transverse momentum. Analysis made on the 4.9 fb−1 of collected data is reported. As no significant excess with respect to Standard Model predictions is observed, the dielectron invariant mass spectrum is interpreted to derive mass limits concerning the existence of new Z' gauge bosons appearing in grand unification theories and the effective sequential standard model (SSM). These limits and those derived by the CMS collaboration are the best ever set on such new bosons.