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

Abstract

The ππ 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 π-π phase shifts. We present some results on the a-meson pole, which is generated dynamically in our model, in nuclear matter. Finally, we analyze the (γ, ππ) reaction on nucleons and nuclei in the kinematical region where the scalar isoscalar ππ scattering amplitude is influenced by the low mass of the σ 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 (γ, π 0 π 0) channel consistent with recent experimental data.