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

Abstract

We analyze the prospects of probing the $CP$-odd $i\tilde{\kappa }\bar{t}{\gamma }^{5}th$ 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 $Wb\to th$ scattering with optimal linear sensitivity to $\tilde{\kappa }$. For the corresponding hadronic process $pp\to thj$ 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.

Updated sensitivity plots due to better data samples