Calorimetry and W mass measurement for future experiments

The precise measure of the W boson mass is an essential input to a crucial test of the overall consistency of the Standard Model, whose failure might reveal the emergence of new physics. With millions of W bosons expected, future experiments will be W boson factories allowing the measurement of the W mass with unparalleled precision. The W mass measurement is discussed in the context of two experiments: the upgrade of CMS at the LHC, the HL-LHC, with a new endcap calorimeter, the HGCal, and a detector for the FCC-ee, a circular post-LHC project. In proton-proton collisions, the precise measurement of the W mass relies on a precise measurement of the hadronic recoil. Its accurate measurement mainly depends on its definition model and detector effects. The recoil definition is improved with machine learning techniques, using a multivariate quantile regression. The effects of the HGCal granularity, acceptance and resolution on the recoil reconstruction are evaluated. This study gives an estimate of the precision that might be reached on the W mass measurement at HL-LHC. Before evaluating the effect of the detector performance on the recoil, the full geometry and parameters had to be tuned and optimised. An accurate fast simulation tool, complement to the full CMS simulation, implemented to undertake such a study, is presented. In electron-positron collisions, the W mass can be determined from the W-pair decay products. The statistical uncertainties on the W mass and width are estimated using a kinematic fit technique in the hadronic and semi-leptonic channels at 162.6 GeV, 240 GeV and 365 GeV. Reaching a statistical precision below the MeV/c² level at all energies for both channels, the W mass measurement becomes limited by the systematic uncertainties. A treatment to reduce the systematic uncertainty coming from QCD effects, the largest source of systematic uncertainty at LEP, and its impact on the statistical uncertainty are also studied.