A method to determine the operator content of perturbed conformal field theories

A method to determine the full structure of the space of local operators of massive integrable field theories, based on the form factor bootstrap approach, is presented. This method is applied to the integrable perturbations of the Ising conformal point. It is found that the content of local operators can be expressed in terms of fermionic sum representations of the characters χ̃~(q) of the Virasoro irreducible representations of the minimal model M3,4. The space of operators factorises into chiral components as Z = Σ χ̃(q) χ̃(q), but with the relation q = q−1.