Fractional charges and the renormalization group in the SU(4) x O(4) string model

We study the renormalization group equations of the gauge couplings in the SU(4)xO(4) ∝ SU(4) x SU(2)L x SU(2)R string model, derived in the context of the free fermionic formulation of the four dimensional superstring. We calculate the effective string unification scale taking into account string threshold corrections and we consider the consequences of the nL and nR fractionally charged states, sitting in the (1,2,1) and (1,1,2) representations correspondingly, of the gauge symmetry of the model. Some of these states become massive at a very high scale, when a number of singlet fields acquire vev's. However, many of them (the precise number depends on the specific choice of the flat direction) remain in the massless spectrum. We consider various cases and find that, for specific choices of flat directions, the physical parameters of the model, like the grand unification scale and the low energy parameters sin2θW and α3, depend only on the difference n-=nL-nR. We study more general cases where remnants of the exotic doublets remain below the SU(4) breaking scale. We also solve the coupled differential system of the renormalization group equations for the gauge and Yukawa couplings and estimate the range of the top quark mass which is found to lie in the range 140 GeV<mt<190 GeV. (orig.)