Study of the $pp \to Z \to \mu^{+}\mu^{-}$ Process at ATLAS: Detector Performance and First Cross-Section Measurement at 7 TeV

This thesis has been focused on the study of the Z-> mu mu process, using data collected during 2010 with the ATLAS experiment. From the detector performance point of view, the Z->mumu process is used to measure muon trigger and reconstruction efficiencies form data. To this aim, a Tag&Probe method has been developed and optimized in the past years using Monte Carlo simulation. The thesis describes both the method, the softwere developed to implement it, and the results obtained using a data sample of 1.3 pbi of integrated luminosity. The computed muon efficiencies have been then used to derive the data/MC scale factor needed for the Z->mumu cross-section measurement. In this measurement the signal efficiency and acceptance is computed used the MC simulation and has to be corrected to match the observed data performances using the scale factors. The measurement of both Z->mumu inclusive and differential cross-sections with respect to the jet multiplicity in the event, performed during the PhD work, is described.