Light Z' in Heterotic String Standard-like Models
  The discovery of the Higgs boson at the LHC supports the hypothesis that the Standard Model provides an effective parameterisation of all subatomic experimental data up to the Planck scale. String theory, which provides a viable perturbative approach to quantum gravity, requires for its consistency the existence of additional gauge symmetries beyond the Standard Model. The construction of heterotic--string models with a viable light $Z^\prime$ is, however, highly constrained. We outline the construction of standard--like heterotic--string models that allow for an additional Abelian gauge symmetry that may remain unbroken down to low scales. We present a string inspired model, consistent with the string constraints. 
P. Athanasopoulos, A.E. Faraggi and V.M. Mehta
hep-th hep-ph