Study of the Spin/CP properties of the Higgs coupling to W bosons with ATLAS at the LHC

The Standard Model does not explain the observed value of the matter-antimatter asymmetry in the Universe, which requires, among other things, additional sources of violation of the charge-parity (CP) symmetry. CP-odd components in the interactions of the Higgs boson with other particles would be a candidate for such a source and are still allowed within experimental uncertainties. The main goal of this thesis is the search for CP-odd components in the interaction between the Higgs boson and W bosons (HWW) in associated production of a W boson and a Higgs boson (WH). This production mode was chosen since it allows measuring the HWW interaction without assumptions on the HZZ interaction, not possible in other production modes such as vector boson fusion. The first step was to estabilish a precision measurement of WH production, done in the context of the ATLAS precision measurement of WH and ZH production using the full LHC Run 2 dataset, presented in this thesis. Despite its very high precision, this measurement is not sensitive to CP-odd components. It is extended into the first ATLAS search for CP-odd components in WH production, presented in this thesis, achieving a precision competitive with the world best results. In order to improve the sensitivity of future iterations of this search, a comparison of a machine-learning-based optimal observable with traditional methods using simulated data is also presented in this thesis, demonstrating the potential of such an observable. Jet triggers are essential for the precision of jet calibrations, used in approximately 60% of all ATLAS analyses. Concurrently with the physics analyses described above, the performance of jet reconstruction at the ATLAS software trigger for LHC Run 3 was studied for this thesis. These studies were first performed using simulated data and validated successfully with first data from Run 3.