Calibration of the electromagnetic barrel calorimeter. Identification of the tau leptons and search for a Higgs boson in the channel qqH → qq ττ in the Atlas experiment at LHC

The Standard Model is the theory which describes the fundamental interactions most accurately. However, the Higgs mechanism and its associated boson have not yet been discovered. The ATLAS electromagnetic calorimeter will play an important role in its discovery if it exists. In the first part of this work, a final mapping of all barrel electromagnetic calorimeter cells, and in particular the problematic ones, was made. Then, the code for the calorimeter calibration was migrated into the ATLAS software environment (ATHENA), where it was tested and validated with the 2004 test beam data. In this code, the optimal filtering coefficients, which enable to reconstruct the energy deposited in the calorimeter while minimizing the electronic and pile-up noises, are calculated. For this, a model was developed to predict the physics signal waveform from the calibration waveform. In a third part, two algorithms for reconstructing and identifying τ leptons in their hadronic decay mode were studied and compared. Finally in a fourth part, one amongst the most important Standard Model Higgs production and decay channels was investigated, namely the weak boson fusion production followed by the Higgs decay into a tau lepton pair, for a low mass Higgs (115 < mHiggs < 145 GeV/c2). This study was performed for 30 fb-1 of integrated luminosity using fast and fully simulated data. A study of the dominant background Z + n jets (n ≤ 5) was also performed. (author)