Improvement of the muon spectrometer and precision W-boson physics in ATLAS

This thesis presents the work on the production and the qualification of the ATLAS NSW Micromegas Large Modules 1 (LM1), as well as the measurements of the W-boson transverse momentum and the W-boson mass using low pile-up data recorded by ATLAS at center-of-mass energies of √s =5.02 and 13 TeV. Detailed protocols and results of the cosmic bench characterization for the LM1 Micromegas modules are documented, including the preliminary tests of the improved working gas mixture which was in the end adopted for the Run 3 data taking. The physics modelling and detector calibration for the W-boson analyses using the low pile-up data are described. The data-driven estimation of the multijet background is presented, emphasizing the improvements with respect to the procedures used in the previous analyses. The measurement of the W-boson transverse momentum achieves a remarkably high precision and will become a valuable input to the W mass analysis. Based on a profile likelihood fit, the framework of statistical analysis for the low pile-up W mass measurement has been studied and optimized. Benefiting from the detector calibration derived from the measurement of the W-boson transverse momentum, the low pile-up W mass analysis is expected to become a proof-of-concept measurement for the precise determination of the Wboson mass using future low pile-up datasets in ATLAS. Also, a coherent approach of uncertainty decomposition for profile likelihood fits is developed and illustrated for the first time.