ar X iv : h ep - p h / 96 02 28 3 v 1 1 3 Fe b 19 96 KEK TH - 469 KEK preprint 95 - 205 February 1996

It has been more than twenty years since the basic idea of the standard model(SM) of elementary particle theory was proposed. This theory is based on two fundamental principles, i.e. the gauge principle and the Higgs mechanism. From recent experiments it is now clear that strong and electroweak interactions are described by an SU(3)×SU(2)×U(1) gauge theory. On the other hand, little is known about mechanism of the electroweak symmetry breaking. To clarify dynamics behind this symmetry breaking in the SM is the primary remaining objective and the most important physics motivation of future experiments at both LHC and ee linear colliders. Exploring the Higgs sector is important not only because the Higgs particle is the only ingredient still missing from the SM, but also because this will be a key to the physics beyond the SM. In the minimal SM the Higgs sector consists of one Higgs doublet, and the only free parameter is the Higgs-boson mass. A heavy Higgs boson corresponds to a large self-coupling constant, and a light Higgs boson suggests that the dynamics of the Higgs sector is well described perturbatively. For many extensions of the Higgs sector a similar relationship holds between the Higgs-boson mass and the strength of the interaction which governs the symmetrybreaking dynamics. For example, if we require perturbative unification of the three gauge coupling constants as predicted in Grand Unified Theories (GUT), the Higgs particle cannot be heavier than about 200 GeV. On the other hand, models like Technicolor predict either a very heavy scalar particle or no particle at all which acts as the Higgs boson.