Top quark pair production near the threshold in e+e- collisions
  It was reported that the NNLO correction to the total cross section near the ttbar threshold in e+e- collisions is as large as the NLO correction to it. In the thesis, we improve the convergence of the perturbative series of the total cross section by choosing appropriate expansion parameters for both m_t and alpha_s. With these two prescriptions, the perturbative convergence is much improved. Correspondingly, the resulting ambiguity for the determination of m_t is estimated to be 0.1GeV.   We also study the effect of all types of anomalous top-quark EDM interactions near the threshold for the first time. We show that one can disentangle these EDMs by the dependence of CP-odd observables on the CM-energy and on the degree of initial state polarization. The sensitivity to chromo-EDM at LC turns out to be better than that at LC in the open top region, but to be worse than that at LHC by a factor 1/10. The sensitivities to electroweak-EDMs near the threshold are similar to those in the open top region at LC. 
Takaaki Nagano