Gauge Symmetry Reduction from the Extra Space S1/Z2

We study a mechanism of symmetry transition upon compactification of a 5-dimensional field theory on $S^1/Z_2$. The transition occurs unless all components in a multiplet of a symmetry group have a common $Z_2$ parity on $S^1/Z_2$. This mechanism is applied to a reduction of SU(5) gauge symmetry in grand unified theory, and phenomenological implications are discussed.Comment: 7 pages, ptptex, no figures, journal versio