Asymptotic factorisation of form factors in two-dimensional quantum   field theory
  It is shown that the scaling operators in the conformal limit of a two-dimensional field theory have massive form factors which obey a simple factorisation property in rapidity space. This has been used to identify such operators within the form factor bootstrap approach. A sum rule which yields the scaling dimension of such operators is also derived. 
G. Delfino, P. Simonetti and J.L. Cardy