Widely advertised as a discovery machine, the LHC proton collider at CERN will also provide large samples of standard particles, allowing the final, most precise experimental determination of some fundamental parameters of particle physics theory. Together with the expected discoveries, these measurements will allow to elucidate the dynamics of electroweak symmetry breaking. The aim of the present project is to organize and realize a precise measurement of the W resonance parameters, and in particular the W boson mass with a target precision below 0.01%. Most LHC measurements aim at a precision of order 1%, and can be performed on a time scale of about one year, within a small group of collaborators. The measurement of the W boson mass is a special case, as its aimed precision requires perfect understanding of the experimental and physical environment. It therefore requires careful planning. Intermediate measurements of standard processes need to be performed, synchronized, and will play a key role in the final result. The primary tools serving this purpose are the study of the production properties of the J/Psi, W and Z particles in the uncertain LHC environment. These studies will allow to understand the performance of the ATLAS detector and to constrain the dynamics of proton-proton interactions, which represent a major source of uncertainty. Once the above properties are firmly established, the distributions sensitive to the W boson mass can be precisely predicted, and the fundamental parameters can be determined. The coordinator of this project has recognized expertise on electroweak physics, and has led the ATLAS Collaboration's research in this field since 2007. As argued above, the preparation of the measurement of mW requires to build and coordinate a team devoted to the realization of this difficult measurement over the next years. The present grant would provide a unique opportunity to gather all necessary conditions for the success of this project.
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