Scaling limit of the one-dimensional XXZ Heisenberg chain with easy axis anisotropy

We construct the scaling limit of the easy axis XXZ chain. This limit is a subtle combination of approaching the isotropic point, and letting the lattice spacing to zero to obtain a continuous model with a finite mass gap. We give the energy difference between the two lowest energy states (the two `vacua') and analyze the structure of the excitation spectrum of the limiting model. We find, that the excitations form two sets corresponding to the two vacua. In both sets the dressed particles are described by Bethe Ansatz like equations (higher level Bethe Ansatz), and the two sets can be distinguished through a parameter entering into these secular equations. The degenerations in the spectrum can be interpreted as originating from an SU(2) symmetry of the dressed particles. The two particle scattering matrices obtained from the secular equations are consistent with this symmetry, and they differ in an overall sign in the two sectors.