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

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\(\mathrm{fb}^{-1}\).