Gauge-Higgs unification at $e^+ e^-$ linear colliders

In gauge-Higgs unification the 4D Higgs boson appears as a part of the fifth dimensional component of gauge potentials, namely as a fluctuation mode of the Aharonov-Bohm phase in the extra dimension. The $SO(5) \times U(1) \times SU(3)$ gauge-Higgs unification gives nearly the same phenomenology as the standard model (SM) at low energies. It predicts KK excited states of photon, $Z$ boson, and $Z_R$ boson ($Z'$ bosons) around 7 - 8 TeV. Quarks and leptons couple to these $Z'$ bosons with large parity violation, which leads to distinct interference effects in $e^+ e^- \rightarrow \mu^+ \mu^-, q \, \bar q$ processes. At 250 GeV ILC with polarized electron beams, deviation from SM can be seen at the 3 - 5 sigma level even with 250 fb$^{-1}$ data, namely in the early stage of ILC. Signals become stronger at higher energies. Precision measurements of interference effects at electron-positron colliders at energies above 250 GeV become very important to explore physics beyond the standard model.