Étude des performances du spectromètre à muons du détecteur Atlas au LHC, des cosmiques aux collisions. Mesure de la section efficace de production des dibosons WZ.

ATLAS is one of the four experiments at the Large Hadron Collider (LHC), located at CERN, Geneva. As the LHC only delivered its first collisions in December 2009, at an energy of 7 TeV in the centre-of-mass, ATLAS recorded millions of cosmic events in 2008 and 2009 in order to better understand the detector. The first part of this PhD thesis deals with these cosmic events in order to estimate the muon spectrometer performances, particularly its efficiency and resolution. Next, using 7 TeV collisions, the efficiency is determined using a “Tag and Probe” method on Z events decaying into muons. In addition, the missing transverse energy is studied and a clear improvement of its resolution is achieved through a better treatment of the muons. Finally, muons, missing transverse energy, but also electrons, are used to estimate the production cross-section of WZ dibosons. Event selection, backgrounds estimation and systematics errors are provided. A computation of the WZ cross-section using 1.02 fb−1 of data and a first estimation of limits on anomalous triple gauge couplings are also given.