GUT ON ORBIFOLDS -- Dynamical Rearrangement of Gauge Symmetry --

Grand unified theory defined on higher-dimensional orbifolds provides a new way to solve the hierarchy problem. In gauge theory on an orbifold many different sets of boundary conditions imposed at orbifold fixed points (branes) are related by large gauge transformations, forming an equivalent class of boundary conditions. Thanks to the Hosotani mechanism the physics remains the same in all theories in a given equivalent class, though the symmetry of boundary conditions differs from each other. Quantum dynamics of Wilson line phases rearranges the gauge symmetry. In the nonsupersymmetric SU(5) model the presence of bulk fermions leads to the spontaneous breaking of color SU(3). In the supersymmetric model with Scherk-Schwarz SUSY breaking, color SU(3) is spontaneously broken even in the absence of bulk fermions if there are Higgs multiplets