Although the experimental data prefer the former light Higgs scenario, there is logical possibility that Nature had taken the latter path. Technicolor scenario, which is based on a new strong interaction, belongs to this class, solving the naturalness problem by setting the cutoff just above the TeV scale. The elementary Higgs field is replaced by the Nambu-Goldstone bosons associated with a dynamical chiral symmetry breaking in the techni-fermion sector. Unlike the early models of supersymmetry, however, all the early models in this category have been excluded experimentally and the remaining models have lost most of the original beauty that motivated the scenario. Nevertheless, if no light Higgs boson is found, we will have to abandon the former scenario and seriously confront this latter possibility. In this sense, this is the crucial branch point to decide the future direction of high energy physics and the JLC can clearly show us which way to take by unambiguously testing the existence of the light Higgs boson as described in Chapter 2. If we are to take the latter path, we will need to scrutinize the W and Z bosons as well as the top quark in great detail to spot any deviation from the Standard Model in order to get insight into the underlying dynamics that is responsible for the spontaneous breaking of the electroweak gauge symmetry. The JLC's ability in such measurements are elucidated in Chapters 4 and 6.
The study of the top quark has, however, fundamental importance in its own right as described in Chapter 4. First of all, a precise measurement of its mass, MeV, is found possible at the threshold thanks to the recent progress in the nonrelativistic QCD. The large width of the top quark acts as an infrared cutoff to the QCD interaction, allowing us to make definite theoretical predictions using perturbative QCD. This remarkable feature provides a new and clean test of perturbative QCD as well as a precise measurement of the strong coupling constant . Both of these measurements together with the W mass determination discussed in Chapter 6 will be indispensable when one tries to probe the physics beyond the Standard Model from the analysis of the radiative corrections. Search for possible CP violation in the top quark system also deserves special mention since its discovery immediately signals physics beyond the Standard Model.
Finally, speaking of the physics beyond the Standard Model that may put the cutoff scale just above the weak scale, we cannot but mention the recent remarkable proposals that have literally added extra dimensions to the possible scenarios. These extra spatial dimensions may give rise to new states that would appear as Z' bosons or spin-2 resonances, depending on the models. In the brane world scenarios, which embed our world as a four-dimensional membrane in a space with higher dimensions, gravity would become strong at TeV energies. In these models, gravitons could be radiated into the bulk and would leave missing energy signals for the process like . It is also possible that the effect of the virtual graviton exchange would show up as a deviation from the Standard Model. The sensitivity of the 500 GeV linear collider is expected to reach a few TeV, which seems enough to find some positive signal if the extra dimensions are somehow related to the naturalness problem.
We have seen above that there are many possibilities and we do not know which way to take for sure, but one thing is clear. Whatever new physics lies beyond the Standard Model, the key to open it is the understanding of the electroweak symmetry breaking, and the clean environment, high luminosity, and the large beam polarization at the JLC will give us a definite answer for it, thereby leading us to make an entirely new step towards the deeper understanding of Nature.