Evaluation of the $\pi\pi$ scattering amplitude in the $\sigma$-channel   at finite density
  The $\pi\pi$ scattering amplitude in the $\sigma$-channel is studied at finite baryonic density in the framework of a chiral unitary approach which successfully reproduces the meson meson phase shifts and generates the $f_0$ and $\sigma$ resonances in vacuum. We address here a new variety of mechanisms recently suggested to modify the $\pi\pi$ interaction in the medium, as well as the role of the $s-$wave selfenergy, in addition to the $p-$wave, in the dressing of the pion propagators. 
D. Cabrera, E. Oset and M.J. Vicente Vacas