A Non-Perturbative Treatment of the Pion in the Linear Sigma-Model
  Using a non-perturbative method based on the selfconsistent Quasi-particle Random-Phase Approximation (QRPA) we describe the properties of the pion in the linear $\sigma$-model. It is found that the pion is massless in the chiral limit, both at zero- and finite temperature, in accordance with Goldstone's theorem. 
Z. Aouissat, G. Chanfray, P. Schuck and J. Wambach