We study consequences of CP violation in the $ht\bar{ t}$ Yukawa coupling through the process $e^+e^- \to$ $h(125)t\bar{ t}$. The helicity amplitudes are calculated in the $t\bar{ t}$ rest frame, where the initial $e^+e^-$ current and the final Higgs boson have the same three-momentum. CP-violating asymmetries appear not only in the azimuthal angle between the $e^+e^-$ plane and the $t\bar{ t}$ plane about the Higgs momentum direction, but also in the correlated decay angular distributions of $t$ and $\bar{ t}$. Complete description of the production and decay angular distributions are obtained analytically, including both leptonic and hadronic decays of $t$ and $\bar{ t}$. We study the ultimate sensitivity to the CP-violating $ht\bar{ t}$ coupling at a few center-of-mass energies. Our analysis shows that the possibility of discovering CP violating $ht\bar{ t}$ coupling improves significantly by studying $t\bar{ t}$ decay angular correlations, and more importantly, by increasing its energy upgrade target from $\sqrt{ s} = 500$ GeV to 550 GeV.
Categories: hep-ph