Mesure de la production de di-bosons WZ auprès du LHC avec l'experience ATLAS

This thesis is performed in the frame of the ATLAS experiment at the LHC. A first part of the work presented in this document consists on the time calibration of the ATLAS Liquid Argon (LAr) calorimeter. The control of the time alignment of the calorimeter is important for the goodness of the quality of the energy reconstructed in the calorimeter. The results presented in this thesis have allowed an improvement of 30% of the global time resolution of the LAr calorimeter. The Standard model of particle physics predicts, during proton collisions, the production of the $W$ and $Z$ weak bosons as a pair due to the interaction of a quark with an anti-quark. The diboson production can be sensitive to the couplings between vector bosons. An anomalous deviation of these couplings from the prediction of the SM would point to the presence of new physics. The use of the full statistics of the 2012 ATLAS data allowed us to increase the precision of the measurement of these couplings compared to previous results based on smaller datasets. This thesis presents therefore the measurement of the $WZ$ dibosons production cross section using the full 2012 data collected by the ATLAS experiment from the $p-p$ collisions at the LHC at a center-of-mass energy of 8 TeV. Also, with the available statistics the ratio of the production cross sections of $W^+Z$ and $W^-Z$ events were measured. This measurement was not performed previously using the 2011 data due to a lack of statistics. Finally, measurements of the normalized differential cross section as a function of four kinematic variables were also performed. The precision on the measured integrated cross section is 5.5% which is reached mainly by the reduction of the statistical uncertainty by 55% with respect to the previous ATLAS results. Therefore, the order of magnitude of the experimental uncertainties on the measurement started to approach that of the theoretical predictions. This is promising for future measurements at the LHC as with higher statistics the experimental precision is expected to overcome the theoretical one.