Studio delle proprietà degli eventi W+jets con l’esperimento ATLAS ad LHC

The aim of this work is the study of $W$ boson production in association with jets using $\sqrt{s} = 7$ TeV proton-proton collisions of the 2011 ATLAS dataset with an integrated luminosity of 4.64 fb$^{-1}$. This thesis describes the event's selection of $W$ decaying to electrons and muons and the main sources of background in both channels focusing on QCD multijet and $t\bar{t}$. They represent a large fraction of background to both electron and muon channels: QCD is relevant at low jet multiplicity while the background from semileptonic top decays becomes very important at high jet multiplicity. Moreover, because of few statistics in MC samples and large uncertainties on multijet and $t\bar{t}$ cross-sections, these backgrounds are estimated using data-driven methods. The control sample selection and the normalisation of QCD multijet and $t\bar{t}$ templates are discussed including an investigation of the QCD template bias in muon channel and correlation studies between the top templates and data. Then, the evaluation of all the systematic uncertainties derived at detector level is described. The comparison between experimental data and theoretical predictions will be provided for a few distributions, such as transverse momentum and rapidity of the leading jet, angular distance between the two leading jets and inclusive event quantities (e.g. $S_T$ and $H_T$). These studies are a critical ingredient towards the measurement of the $W+$jets cross-section and will contribute to the ATLAS forthcoming publication of the $W+$jets cross-section measurement.