Nature of the σ meson as revealed by its softening process

The ππ scattering is studied in two-flavor chiral models with a finite pion mass to investigate the nature of the σ meson which is observed as the lowest scalar–isoscalar resonance. We compare several models with different origins of the σ meson, such as the chiral partner of the pion and the dynamically generated ππ molecule. We find that the dynamically generated σ meson exhibits a novel pattern of the threshold enhancement reflecting the s-wave nature of the resonance, which is qualitatively different from the softening of the chiral partner introduced as a bare field. This behavior around the threshold energy region is universal as far as the bare σ pole stays away from the threshold throughout the symmetry restoration process. On the other hand, for mπ=0, the dynamically generated σ behaves similarly to the chiral partner in the symmetry restoration limit, implying the possibility of the dynamically generated chiral partner.