6D gauge-Higgs unification on T^2/Z_N with custodial symmetry
  We investigate the gauge-Higgs unification models compactified on T^2/Z_N that have the custodial symmetry. We select possible gauge groups, orbifolds and representations of the matter fermions that are consistent with the custodial symmetry, by means of the group theoretical analysis. The best candidate we found is 6D SU(3)_C x U(4) gauge theory on T^2/Z_3 and the third generation quarks are embedded into bulk fermions that are the symmetric traceless rank-2 tensor of SO(6). 
Yoshio Matsumoto, Yutaka Sakamura
hep-ph hep-th