Summing planar diagrams by an integrable bootstrap

Correlation functions of matrix-valued fields are not generally known for massive renormalized field theories. We find the large-N limit of form factors of the (1 + 1)-dimensional sigma model with SU(N) x SU(N) symmetry. These form factors give a correction to the free-field approximation for the N 1 Wightman function. The method is a combination of the 1/N expansion of the S matrix and Smirnov's form-factor axioms. We expand the renormalized field in terms of a free massive bosonic field as N -> infinity.