STRUCTURE OF THE σ MESON AND THE SOFTENING

We study the structure of the σ meson, the lowest-lying resonance of the ππ scattering in the scalar-isoscalar channel, through the softening phenomena associated with the partial restoration of chiral symmetry. We build dynamical chiral models to describe the ππ scattering amplitude, in which the σ meson is described either as the chiral partner of the pion or as the dynamically generated resonance through the ππ attraction. It is shown that the internal structure is reflected in the softening phenomena; the softening pattern of the dynamically generated σ meson is qualitatively different from the behavior of the chiral partner of the pion. On the other hand, in the symmetry restoration limit, the dynamically generated σ meson behaves similarly to the chiral partner.