IMPLICATIONS OF ABELIAN EXTENDED GAUGE STRUCTURES FROM STRING MODELS

Within a class of superstring vacua which have an additional nonanomal ous U(1)' gauge factor, we address the scale of the U(1)' symmetry bre aking and constraints on the exotic particle content and their masses. We also show that an extra gauge U(1)' provides a new mechanism for g enerating a naturally small effective mu. term. In general, existing m odels are not consistent with all phenomenological constraints; howeve r, they do provide a testing ground to address the above issues, yield ing a set of concrete scenarios. Under the assumptions that the sponta neous U(1)' breaking takes place in the observable sector and that the supersymmetry-breaking scalar mass square terms are positive at the s tring scale, the breaking of U(1)' symmetry is radiative. It can take place when the appropriate Yukawa couplings of exotic particles are of order 1, which occurs for Z(2)xZ(2), fermionic orbifold constructions at symmetric points of moduli space. The Z' mass is either of O(M(Z)) , when the symmetry breaking is due to a single standard model singlet , or of a scale intermediate between the string and electroweak scales , determined by the radiative corrections (or by competing nonrenormal izable operators), when the breaking is due to two or more mirrorlike singlets. In the former case, the M(Z')/M(Z) hierarchy achievable with out excessive fine-tuning is within future experimental reach.