Abstract :
For a large class of integrable quantum field theories we show that the S-matrix determines a space of fields which decomposes into subspaces labeled, besides the charge and spin indices, by an integer k. For scalar fields k is non-negative and is naturally identified as an off-critical extension of the conformal level. To each particle we associate an operator acting in the space of fields whose eigenvectors are primary (image) fields of the massive theory. We discuss how the existing results for models as different as image, sine-Gordon or Ising with magnetic field fit into this classification.
