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

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