The Linear Sigma-Model in the 1/N-Expansion via Dynamical Boson Mappings   and Applications to $\pi\pi$-Scattering
  We present a non-perturbative method for the study of the O(N+1)-version of the linear sigma-model. Using boson-mapping techniques, in close analogy to those well-known for fermionic systems, we obtain a systematic 1/N-expansion for the Hamiltonian which is symmetry-conserving order by order. The leading order for the Hamiltonian is evaluated explicitly and we apply the method to $\pi\pi$-scattering, in deriving the T-matrix to leading order. 
Z. Aouissat, P. Schuck and J. Wambach