Leptophobic U(1)'s and the R(b)-R(c) anomalies

In this paper. we investigate the possibility of explaining both the R(b) excess and the R(c) deficit reported by the CERN LEP experiments through Z-Z' mixing effects. We have constructed a set of models consistent with a restrictive set of principles: unification of the standard model (SM) gauge couplings, vector-like additional matter, and couplings which are both generation independent and leptophobic. These models are anomaly-free, perturbative up to the GUT scale, and contain realistic mass spectra. Out of this class of models, we find three explicit realizations which fit the LEP data to a far better extent than the unmodified SM or MSSM and satisfy all other phenomenological constraints which we have investigated. One realization, the eta model coming from E(6), is particularly attractive, arising naturally from geometrical compactifications of heterotic string theory. This conclusion depends crucially on the inclusion of a U(1) kinetic mixing term, whose value is correctly predicted by renormalization group running in the E(6) model given one discrete choice of spectra.