Plateaux transitions from S-matrices based on invariant field theories

Abstract A scattering description is proposed for a boundary perturbation of a c =1 SL(2, Z ) invariant conformal field theory. The bulk massless S -matrices are of the form of Zamolodchikov's staircase model. Using the boundary version of the thermodynamic Bethe ansatz, we show that the boundary free energy goes through a series of integer valued plateaux as a function of system size.