4.9 Summary

In this chapter we have collected the research results of the ACFA Top Quark Working Group during the period 1998-2001.

Physics studies in the $t\bar{t}$ threshold region will be one of the major subjects of future experiments at JLC. Accordingly there have been extensive studies in the working group. Especially theoretical understandings of the physics in this region have developed remarkably. Also, we have initiated detailed Monte Carlo studies focusing in this energy region and they are still on-going. The most important outcome up to now is the establishment of a future prospect that we will be able to measure the top quark $\overline{\rm MS}$ mass precisely ( $\delta m_t \approx 50$-150 MeV). Furthermore, we have clarified that a unique study of various properties of the top quark will be possible.

Studies of the top quark properties in the energy region far above the threshold extend to wide varieties. A possibility of the top quark mass determination from the dilepton channel, under a fairly clean experimental environment, is demonstrated. The Monte Carlo study on the measurement of the top quark Yukawa coupling has put a benchmark of achievable accuracy of this coupling. The theoretical study on probing the anomalous $t\bar{t}H$ coupling has elucidated an interesting possibility of probing the electroweak symmetry breaking physics through the top quark interactions with the Higgs boson. Even through detailed study of the Standard-Model vertices of the top quark, which can be extracted from the decay processes of the top quark, JLC will enable us to provide hints to new physics. The study of CP-violating interactions of the top quark is interesting in its own right. Sensitivity studies to these interactions have been performed and reviewed. Also, the top quark can be used as a probe for R-parity violating SUSY interactions.

Thus, we have shown that from the detailed studies of the top quark physics at JLC, we will be able to study deeply into the the Standard-Model physics and beyond.