Nature of the $\sigma$ 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 m π = 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.

11.30.Rd
12.39.Fe
11.55.Fv
13.75.Lb
sigma meson
Chiral dynamics
Softening
Chiral partner
Hadronic molecule
σ meson

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