This two-year award supports U.S.-France cooperative research in theoretical physics between Tomasz Taylor of Northeastern University and Ignatios Antoniadis of the Ecole Polytechnique, Paris, France. The investigators will study a theory of particle physics based on four-dimensional superstrings. Their principal objective is to determine the derivation of actions that describe the physics of string excitations at low energies. They will achieve this by computing the relevant string scattering amplitudes. The results will be applied to studies of non- perturbative effects in superstring theory and to the problem of supersymmetry breaking. Both investigators have been involved in research on supergravity and superstring theory. Dr. Taylor brings to this collaboration his background in the phenomenological aspects of superstring theory and expertise on supersymmetry breaking and space-time symmetries. This is complemented by Dr. Antoniadis expertise on superstring modes and theories and his involvement in the first constructions of four-dimensional string theories and their properties. Their combined effort will contribute to our understanding of superstring theory and string/particle interface. Superstring theory is important to the fields of particle physics, modern mathematics and condensed matter physics. It provides a framework of unification for fundamental interactions of nature and holds great promise as a theory of quantum gravitation.