Leptophobic Z' from superstring derived models

It was recently suggested that the reported anomalies in Rb and Rc can be interpreted as the effect of a heavy vector boson that couples to quarks and is universally decoupled from leptons. We examine how an extra gauge boson with this property can arise from superstring derived models. In a specific three generation model we show that the U(1)B-L symmetry combines with the horizontal flavor symmetries to form a universal leptophobic U(1) symmetry. In our model there is an enhancement of the color gauge group from twisted sectors. The enhancement occurs after the breaking of the unifying gauge symmetry by ''Wilson lines.'' The leptophobic U(1) symmetry then becomes a generator of the color SU(4) gauge group. We examine how similar symmetries may appear in other string models without the enhancement. We propose that if the current LEP anomalies persist it may be evidence for a certain class of un-unified superstring models. 15 refs., 4 tabs