SU(2)XSU(2)-INVARIANT SCATTERING MATRIX OF THE HUBBARD-MODEL

We consider the one-dimensional half-filled Hubbard model. We show tha t the excitation spectrum is given by the scattering states of four el ementary excitations, which form the fundamental representation of SU( 2) x SU(2). We determine the exact two-particle scattering matrix, whi ch is a solution of the Yang-Baxter equation and reflects the SO(4) sy mmetry of the model. The results for the repulsive and attractive Hubb ard model are related by an interchange of spin and charge degrees of freedom.