FINITE-SIZE EFFECTS IN THEORIES WITH FACTORIZABLE S-MATRICES

We study the energy spectrum of (1 + 1)-dimensional perturbed conformal field theories defined on the cylinder. The finite-size dependence of the two-particle levels allows a direct numerical measurement of the elastic S-matrix which we compare with the conjectured minimal S-matrix for several perturbations of minimal models. We discuss the simplifications that integrability imposes on the spectrum above threshold. In particular, the ultraviolet limit of the elastic phase shift of two lightest particles is related in a simple way to scaling dimensions of the conformal field theory.