W Boson Production in Association with Hadronic Jets at ATLAS

The Large Hadron Collider (LHC) offers unprecedented opportunities to explore unknown kinematic regions and discover new Physics. Its discovery capabilities, however, strongly depend on our understanding of the Standard Model. Leptonic decays of the W boson produced in association with jets are one of the dominant backgrounds to Physics beyond the Standard Model, particularly Supersymmetry. These processes can also serve as testing ground for the theory of strong interactions. Their study is therefore of great importance at the LHC. The aim of this thesis is to prepare the essential tools for an early data measurement of W(enu) + jets processes, assuming an integrated luminosity of 100 pb^{-1}. Based on Monte Carlo simulations, several aspects of the analysis are discussed, from the treatment of backgrounds, to the calculation of electron efficiencies and acceptances. The focus is on data-driven techniques, which are going to be crucial, particularly in the early-data phase, to minimise dependence on Monte Carlos. A data-driven method for the extraction of electron efficiencies using Zee events is thoroughly studied. The expected statistical and systematic uncertainties are evaluated and the systematic bias on the W(enu) + jets cross-section is assessed. Finally, the major sources of systematic uncertainties are treated and the experimental prospects for measuring the W(enu) + Njets absolute cross-section and the ratios (W + Njets +1)/(W + Njets) and (W + Njets)/(Z + Njets) (with Njets = 1, 2) are presented. It is suggested that the ratios are to be preferred as robust measurements for the early data scenario and it is discussed what aspects of the analysis are the most critical in each case.