Heterotic String Models with Perturbatively Broken Supersymmetry?

By employing our standard analysis of flat directions we present a quasi-realistic three generation free fermionic heterotic string model in which an F- and D- flat solution does not appear to exist at least to eighth order in the superpotential. Our analysis suggests that stringent flat directions may not exist to any order. Bose-Fermi degeneracy of the string spectrum implies that the one-loop partition function and hence the one-loop cosmological constant vanishes in the model. Hence this model may represent the first known example with vanishing cosmological constant and perturbatively broken supersymmetry. We discuss the distinctive properties of the internal free fermion boundary conditions that may correspond to a large set of models that share these properties.