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Spinor-vector duality and light Z' in heterotic string vacua

We discuss the construction of heterotic--string models that allow for the existence of an extra $Z^\prime$ at low scales. One of the main difficulties encountered is that the desired symmetries tend to be anomalous in the prevailing three generation constructions. The reason is that these models utilise the symmetry breaking pattern $E_6\rightarrow SO(10)\times U(1)_ζ$ by GGSO projections. Consequently, $U(1)_ζ$ becomes anomalous. The spinor--vector duality that was observed in the fermionic $Z_2\times Z_2$ orbifold compactifications is used to construct a phenomenological three generation Pati--Salam heterotic--string model in which $U(1)_ζ$ is anomaly free and therefore can be a component of a low scale $Z^\prime$. The model implies existence of matter states at the $Z^\prime$ breaking scale, which are required for anomaly cancelation. Moreover, the string model gives rise to exotic states, which are $SO(10)$ singlets but carry exotic $U(1)_ζ$ charges. These states arise due to the breaking of $E_6$ by discrete Wilson lines and provide natural dark matter candidates. Initial indications suggest that the existence of additional gauge symmetries at the TeV scale may be confirmed in run II of the LHC experiment.