Nonrenormalizable superpotential in four-dimensional string models

A construction of an effective nonrenormalizable superpotential has been di scussed within the approach of a four-dimensional heterotic superstring wit h free fermions on the world sheet. On the basis of N = 2 superconformal th eory, useful selection rules that take into account all global symmetries h ave been obtained within the formalism of covariant vertices and the ghost- picture-changing operation. The results have been applied to the model feat uring the effective observable gauge group U(5) x U(3)(H), which includes t he non-Abelian horizontal group and which describes (3 + 1) generation. The full superpotential of the model up to sixth order inclusive has been obta ined, and its coupling constants have been calculated. The quark-lepton mas s spectrum of the model has been discussed with allowance for contributions from nonrenormalizable terms. (C) 2000 MAIK "Nauka/lnterperiodica ".