$\cal{CP}$-sensitive simplified template cross-sections for $t\bar t H$

Abstract: The $\cal{CP}$ structure of the Higgs boson is a fundamental property which has not yet been constrained with high precision. $\cal{CP}$ violation in the Yukawa coupling between the Higgs boson and top quark pair can be probed directly at the Large Hadron Collider by measuring top-quark-associated Higgs production. Multivariate analysis techniques commonly developed so far by the experiments are designed for a specific signal model and, therefore, complicate reinterpretations and statistical combinations. With this motivation in mind, we propose a $\cal{CP}$-sensitive extension of the simplified template cross-section (STXS) framework. Considering multiple Higgs decay channels, we perform an in-depth comparison of $\cal{CP}$-sensitive observables and combinations thereof. Our resulting proposal is to extend the existing binning in the transverse momentum of the Higgs boson $p_{T,H}$ by either the pseudorapidity difference of the two top-quarks $\Delta \eta_{t\bar t}$, or a variable that is based on the top quark momenta, namely $b_2$ or the Collins-Soper angle $|\cos\theta^*|$. We demonstrate that this variable selection provides close to optimal sensitivity to the $\cal{CP}$ mixture in the top Yukawa coupling for an integrated luminosity of $300\mathrm{fb}^{-1}$, by comparing it to the results of a multivariate analysis. Our results also suggest a benefit of the two-dimensional STXS extension at 3000$\mathrm{fb}^{-1}$.