Universal extra dimensions and b -> s gamma

We analyze the effect of flat universal extra dimensions (i.e., extra dimen sions accessible to all SM fields) on the process b --> s gamma. With one H iggs doublet, the dominant contribution at one-loop is from Kaluza-Klein (K K) states of the charged would-be-Goldstone boson (WGB) and of the top quar k. The resulting constraint on the size of the extra dimension is comparabl e to the constraint from T parameter. In two-Higgs-doublet model II, the co ntribution 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 gam ma and also the constraint on the mass of the charged Higgs from this proce ss is weakened compared to 4D. In two-Higgs-doublet model I, the contributi on 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 t an beta, the constraint on the size of the extra dimensions is stronger tha n in one-Higgs-doublet model and also the constraint on the mass of the cha rged Higgs is stronger than in 4D. (C) 2001 Published by Elsevier Science B .V.