The Standard Model (SM) predicts the Higgs boson to be a CP-even (scalar) particle. Any deviation from a purely CP-even interaction of the Higgs boson with other SM particles would indicate physics beyond the SM.
This text presents a search for CP violating effects in Higgs boson production in association with a W boson, using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider from 2015-2018, corresponding to an integrated luminosity of $139~\text{fb}^{-1}$ at a center-of-mass energy of 13 TeV. The analysis focuses on the Higgs boson decay to bottom quark pairs and the W boson decay to leptons: $WH$, with $H \rightarrow b \overline{b}$ and $W \rightarrow \ell\nu$ ($\ell = e, \mu$).
Fiducial cross-section measurements are performed using the Simplified Template Cross Section (STXS) formalism in bins of an angular observable and the W boson transverse momentum, providing a sensitive probe to CP-violating components in the $HWW$ vertex. The results are interpreted within the Standard Model Effective Field Theory, and constraints are set on the relevant Wilson coefficient in the Warsaw basis, $c_{H\widetilde{W}}$ (CP-odd operator).