Experimental signals of $\sigma$-mesons in nuclei and chiral symmetry

Abstract.

The $\pi\pi$ interaction in the scalar isoscalar channel is studied as a function of the baryonic density in the framework of a chiral unitary approach which successfully reproduces vacuum $\pi$-$\pi$ phase shifts. We present some results on the $\sigma$-meson pole, which is generated dynamically in our model, in nuclear matter. Finally, we analyze the ($\gamma,\pi\pi$) reaction on nucleons and nuclei in the kinematical region where the scalar isoscalar $\pi\pi$ scattering amplitude is influenced by the low mass of the $\sigma$ in nuclei, and thus, presents a large enhancement close to the position of its pole. Indeed, we find, that the final-state interaction of the pions modified by the nuclear medium produces a large shift of strength of the two-pion invariant-mass distribution in the ($\gamma,\pi^0\pi^0$) channel consistent with recent experimental data.