INTEGRABLE PERTURBATIONS OF ZN PARAFERMION MODELS AND THE O(3) SIGMA MODEL

A system of thermodynamic Bethe ansatz equations is suggested for integrable perturbation of the Z(N) parafermion CFT model by the basic parafermion field. Several checks are performed to justify the relevance of the system suggested. In the limit N --> infinity we obtain an infinite TBA system describing the finite size effects in the non-linear O(3) sigma model. Certain modified Z(N) parafermion perturbations are introduced, which are argued to converge at N --> infinity to the O(3) sigma model with the theta-term. For theta = pi we suggest for every N a massless TBA system describing the interpolating renormalization group flow from the ultraviolet Z(N) fixed point to the infrared fixed point, which turns out to be the minimal unitary CFT model M(N + 1).