The Effective Potential and Vacuum Stability within Universal Extra Dimensions

The one-loop effective potential calculated for a generic model that originates from 5-dimensional theory reduced down to 4 dimensions is considered. The cut-off and dimensional regularization schemes are discussed and compared. It is demonstrated that the prescriptions are consistent with each other and lead to the same physical consequences. Stability of the ground state is discussed for a U(1) model that is supposed to mimic the Standard Model extended to 5 dimensions. It has been shown that fermionic Kaluza-Klein modes can dramatically influence the shape of the effective potential shifting the instability scale even by several orders of magnitude.