  We analyze the effect of flat universal extra dimensions (i.e., extra
dimensions accessible to all SM fields) on the process b -> s gamma. With one
Higgs doublet, the dominant contribution at one-loop is from Kaluza-Klein (KK)
states of the charged would-be-Goldstone boson (WGB) and of the top quark. The
resulting constraint on the size of the extra dimension is comparable to the
constraint from T parameter. In two-Higgs doublet model II, the contribution of
zero-mode and KK states of physical charged Higgs can cancel the contribution
from WGB KK states. Therefore, in this model, there is no constraint on the
size of the extra dimensions from the process b -> s gamma and also the
constraint on the mass of the charged Higgs from this process is weakened
compared to 4D. In two Higgs doublet model I, the contribution of the zero-mode
and KK states of physical charged Higgs and that of the KK states of WGB are of
the same sign. Thus, in this model and for small tan beta, the constraint on
the size of the extra dimensions is stronger than in one Higgs doublet model
and also the constraint on the mass of the charged Higgs is stronger than in
4D.
