Enhancement of $\pi A \to \pi\pi A$ Threshold Cross Sections by   In-Medium $\pi\pi$ Final State Interactions
  We address the problem of pion production in low energy $\pi$-nucleus collisions. For the production mechanism we assume a simple model consisting of a coherent sum of single pion exchange and the excitation---followed by the decay into two pions and a nucleon---of the $N^*(1440)$ resonance. The production amplitude is modified by the final state interaction between the pions calculated using the chirally improved J\"ulich meson exchange model including the polarization of the nuclear medium by the pions. The model reproduces well the experimentally observed $\pi A \to \pi\pi A$ cross sections, especially the enhancement with increasing $A$ of the $\pi^+\pi^-$ mass distribution in the threshold region. 
R. Rapp, J.W. Durso, Z. Aouissat, G. Chanfray, O.Krehl, P. Schuck, J.   Speth and J. Wambach