Probing the CP properties of top Yukawa coupling at an e + e − collider

Abstract We study consequences of CP violation in the htt¯ $$ ht\overline{t} $$ Yukawa coupling through the process e+e−→h125tt¯ $$ {e}^{+}{e}^{-}\to h(125)t\overline{t} $$. The helicity amplitudes are calculated in the tt¯ $$ t\overline{t} $$ 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 tt¯ $$ t\overline{t} $$ plane about the Higgs momentum direction, but also in the correlated decay angular distributions of t and t¯ $$ \overline{t} $$. Complete description of the production and decay angular distributions are obtained analytically, including both leptonic and hadronic decays of t and t¯ $$ \overline{t} $$. We study the ultimate sensitivity to the CP-violating htt¯ $$ ht\overline{t} $$ coupling at a few center-of-mass energies. Our analysis shows that the possibility of discovering CP-violating htt¯ $$ ht\overline{t} $$ coupling improves significantly by studying tt¯ $$ t\overline{t} $$ decay angular correlations, and more importantly, by increasing its energy upgrade target from s=500 $$ \sqrt{s}=500 $$ GeV to 550 GeV