Z' Physics from Strings
  Many extensions of the standard model, especially grand unified theories and superstring models, predict the existence of additional Z' bosons and associated exotic chiral supermultiplets. It has recently been argued that for classes of string motivated models with supergravity mediated supersymmetry breaking there are two scenarios for the additional Z's: either the mass is in the accessible range < O(1 TeV), providing a natural solution to the mu problem and implications for the Higgs and sparticle masses and for the LSP; or, when the breaking is associated with a D-flat direction, at an intermediate scale, providing a possible explanation for the hierarchies of quark and charged lepton masses and new possibilities for neutrino masses. Related work, examining the detailed structure of specific perturbative string vacua for D and F-flat directions, surviving U(1)'s and exotics, and effective couplings, is briefly described. 
Paul Langacker