Probing the $CP$ nature of the top quark Yukawa at hadron colliders

TL;DR

This paper explores methods to detect the $CP$-odd component of the top quark Yukawa interaction at current and future hadron colliders, introducing novel observables based on top polarization and momenta of decay products.

Contribution

It develops new $CP$-odd observables for top-Higgs interactions that do not require charge identification of $b$-jets, enhancing sensitivity to $CP$ violation.

Findings

Potential to probe $ ilde angle$ 0.5 at HL-LHC

Potential to probe $ ilde angle$ 0.1 at HE-LHC

New methods for extracting phase-space dependent weights

Abstract

We analyze the prospects of probing the $CP$-odd $i \tilde \kappa \bar t \gamma^5 t h$ interaction at the LHC and its projected upgrades, the high-luminosity and high-energy LHC, directly using associated on-shell Higgs boson and top quark or top quark pair production. To this end we first construct a $CP$-odd observable based on top quark polarization in $W b \to t h$ scattering with optimal linear sensitivity to $\tilde \kappa$. For the corresponding hadronic process $pp \to t h j$ we present a method of extracting the phase-space dependent weight function that allows to retain close to optimal sensitivity to $\tilde{\kappa}$. We project future sensitivity to the signal in $pp \to t(\to \ell \nu b )h(\to b\bar b) j$. Based on these insights we propose novel $CP$-odd observables for top quark pair production in association with the Higgs, $pp \to t \bar t h$, with semileptonically decaying tops and $h\to \bar b b$, that rely solely on measuring the momenta of leptons and $b$-jets from the decaying tops without having to distinguish the charge of the $b$-jets. Among the many possibilities we single out an observable that can potentially probe $\tilde \kappa \sim 0.5$ at the high-luminosity LHC and $\tilde \kappa \sim 0.1$ at high-energy LHC with $2\sigma$ confidence.