A Light Z' Heterotic-String Derived Model
  The existence of an extra Z' inspired from heterotic-string theory at accessible energy scales attracted considerable interest in the particle physics literature. Surprisingly, however, the construction of heterotic--string derived models that allow for an extra Z' to remain unbroken down to low scales has proven to be very difficult. The main reason being that the U(1) symmetries that are typically discussed in the literature are either anomalous or have to be broken at a high scale to generate light neutrino masses. In this paper we use for that purpose the self duality property under the spinor vector duality, which was discovered in free fermionic heterotic-string models. The chiral massless states in the self--dual models fill complete 27 representations of E6. The anomaly free gauge symmetry in the effective low energy field theory of our string model is $SU(4)_C\times SU(2)_L\times SU(2)_R\times U(1)_\zeta$, where $U(1)_\zeta$ is the family universal $U(1)$ symmetry that descends from E6, and is typically anomalous in other free fermionic heterotic-string models. Our model therefore allows for the existence of a low scale Z', which is a combination of $B-L$, $T_{3_L}$ and $T_{3_R}$. The string model is free of exotic fractionally charged states in the massless spectrum. It contains exotic SO(10) singlet states that carry fractional, non--E6 charge, with respect to $U(1)_\zeta$. These non-E6 states arise in the string model due to the breaking of the E6 symmetry by discrete Wilson lines. They represent a distinct signature of the string vacua and cannot arise in E6 Grand Unified Theories. They may provide viable dark matter candidates. 
Alon E. Faraggi, John Rizos
hep-th hep-ph