Form factors for principal chiral field model with   Wess-Zumino-Novikov-Witten term
  We construct the form factors of the trace of energy-momentum tensor for the massless model described by $SU(2)$ principal chiral field model with WZNW tern on level 1. We explain how this construction can be generalized to a class of integrable massless models including the flow from tricritical to critical Ising model. 
P. Mejean and F.A. Smirnov