Non-integrable quantum field theories as perturbations of certain integrable models

We approach the study of non-interable models of two-dimensional quantum field theory as perturbations of the integrable ones. By exploiting the knowledge of the exact S-matrix and form factors of the integrable field theories we obtain the first-order corrections to the mass ratios, the vacuum energy density and the S-matrix of the non-integrable theories. As interesting applications of the formalism, we study the scaling region of the Ising model in an external magnetic field at T similar to T-c and the scaling region around the minimal model M(2,7). For these models, a remarkable agreement is observed between the theoretical predictions and the data extracted by a numerical diagonalisation of their Hamiltonian.