Scattering phases on finite lattices in the broken phase of the   four-dimensional O(4)-\phi^4 theory
  According to a proposal of Luescher it is possible to determine elastic scattering phases in infinite volume from the energy spectrum of two-particle states in a periodic box. We demonstrate the applicability of this method in the broken phase of the four-dimensional O(4) non-linear sigma-model in a Monte Carlo study on finite lattices. This non-perturbative approach also permits the study of unstable particles, the sigma-particle in our case. We observe the sigma-resonance and extract its mass and width. In all scattering channels investigated the results are completely consistent with perturbative calculations. 
M. Goeckeler, H.A. Kastrup, J. Westphalen and F. Zimmermann