Two Pion Production in Nucleon-Nucleon Collisions at Intermediate   Energies
  We have developed a model for the N N --> N N pi pi reaction and evaluated cross sections for the different charged channels. The low energy part of those channels where the pions can be in an isospin zero state is dominated by N* excitation, driven by an isoscalar source followed by the decay N* --> N (pi pi, T=0, s-wave). At higher energies, and in channels where the pions are not in T=0, Delta excitation mechanisms become relevant. 
L. Alvarez-Ruso, E. Oset (Valencia U.), E. Hernandez (Salamanca U.)