On the Form Factors of Relevant Operators and their Cluster Property
  We compute the Form Factors of the relevant scaling operators in a class of integrable models without internal symmetries by exploiting their cluster properties. Their identification is established by computing the corresponding anomalous dimensions by means of Delfino--Simonetti--Cardy sum--rule and further confirmed by comparing some universal ratios of the nearby non--integrable quantum field theories with their independent numerical determination. 
C. Acerbi, G. Mussardo and A. Valleriani