NONRENORMALIZABLE TERMS IN THE FREE FERMIONIC FORMULATION OF 4D-STRINGS

We examine the nonrenormalizable terms in the effective superpotential of four-dimensional free fermionic strings. It is shown that generically such terms are nonvanishing in a large class of models. We discuss the constraints they must satisfy to be nonzero and illustrate this explicitly by evaluating a representative nonrenormalizable term in a particular model, namely the flipped SU(5)×U(1) model. We further point out that this is the only class of models where nonrenormalizable terms can be unambiguously calculated, thus greatly enhancing the predictive power of the theory. © 1990.