Towards string predictions

The aim of superstring phenomenology is to develop the tools and methodolog y needed to confront string theory with experimental data. The first mandat ory task is to find string solutions which reproduce the observable data. T he subsequent goal is to extract potential signatures beyond the observable data. Recently, by studying exact flat directions of non-Abelian singlet f ields, we demonstrated the existence of free fermionic heterotic string mod els in which the SU(3) x SU(2) x U(1)(Y)-chaxged matter spectrum, just belo w the string scale, consists solely of the MSSM spectrum. In this paper we study the possibility that the exact flat directions leave a U(1)(Z ') symm etry unbroken at the Planck scale. We demonstrate in a specific example tha t such unbroken U(1)(Z ') is in general expected to be not of the GUT type but of intrinsic stringy origin. We study its phenomenological characterist ics and the consequences in the case that U(1)(Z ') remains unbroken down t o low energies. We suggest that observation in forthcoming colliders of a Z ', with universal couplings for the two light generations but different co uplings for the heavy generation may provide evidence for the Z(2) x Z(2) o rbifold which underlies the free fermionic models.