Orateur
Description
The structure of the Higgs boson remains one of its least precisely constrained properties. Possible $\mathcal{CP}$ violation in the Yukawa interaction between the Higgs boson and the top quark can be directly probed at the LHC through measurements of top-quark-associated Higgs production ($t\bar{t}H$). Current multivariate analysis strategies are typically optimized for specific signal models, which limits their reinterpretation and combination potential.
To address this, we propose a $\mathcal{CP}$-sensitive extension of the Simplified Template Cross Section (STXS) framework. Considering multiple Higgs decay channels, we perform a detailed comparison of $\mathcal{CP}$-sensitive observables and their combinations. The proposed extension augments the existing Higgs transverse momentum binning, $p_{\mathrm{T},H}$, with additional observables such as the pseudorapidity difference between the top quarks, $\Delta\eta_{t\bar{t}}$, or kinematic variables derived from the top-quark momenta, including $b_2$ and the Collins--Soper angle $|\cos\theta^{*}|$.
This approach achieves near-optimal sensitivity to the $\mathcal{CP}$ admixture in the top Yukawa coupling for an integrated luminosity of $300~\mathrm{fb}^{-1}$, and shows further improvement with a two-dimensional STXS extension at $3000~\mathrm{fb}^{-1}$. Work is ongoing to incorporate a dedicated top-quark definition within the STXS framework to enhance the robustness and interpretability of future measurements.
