In-Medium $\pi-\pi$ Correlations in the '$\sigma$-Meson'-Channel
  The present status of the in-medium invariant $\pi-\pi$ mass distribution in the sigma-meson channel around the threshold is reviewed from the theory side and contrasted with recent experiments on $(\pi,2\pi)$ knock-out reactions. A preliminary investigation indicates that the strongly target-mass dependent invariant mass enhancement of the two pions can be explained theoretically. A more refined reaction theory is needed to confirm this result. In the theoretical description, based on the linear sigma model, emphasis is put on constraints from chiral symmetry. 
P. Schuck, Z. Aouissat, F. Bonutti, G. Chanfray, E. Fragiacomo, N.   Grion and J. Wambach