Enhancement of πA → ππA threshold cross sections by in-medium ππ final state interactions

Abstract

We address the problem of pion production in low-energy π-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 Julich meson exchange model including the polarization of the nuclear medium by the pions. The model reproduces well the experimentally observed πA → ππA cross sections, especially the enhancement with increasing A of the π + π - mass distribution in the threshold region.