Dynamical Gauge-Higgs Unification

Gauge fields and both adjoint and fundamental Higgs fields are unified in gauge theory defined on an orbifold. It is shown how the Hosotani mechanism at the quantum level resolves the problem of the arbitrariness in boundary conditions imposed at the fixed points of the orbifold. The role of adjoint Higgs fields in the standard GUT, which are extra-dimensional components of gauge fields in the current scheme, is taken by the Hosotani mechanism and additional dynamics governing the selection of equivalence classes of boundary conditions. The roles of fundamental Higgs fields, namely those of inducing the electroweak symmetry breaking and giving masses to quarks and leptons, are taken by the Hosotani mechanism and by extra twists in boundary conditions for matter. SUSY scenario nicely fits this scheme. Explicit models are given for the gauge groups U(3) x U(3), SU(5), and SU(6) on the orbifolds M^4 x (S^1/Z_2) and M^4 x (T^2/Z_2).