Narrow Results

We explore the nature of the σ or f₀(600) meson using large N_c technique, assuming that the σ pole dominates low energy physics in the IJ=00 channel. We trace the N_c dependence of the σ pole position using [1,1] Padé approximation of SU(3) chiral amplitude, [1,2] Padé approximation of SU(2) chiral amplitudes, and the PKU parametrization form. We find that in all 3 cases the σ pole moves to the real axis of the complex s-plane. Finally we construct a pedagogical example using the SU(2) Padé amplitude to show that when the analyticity property of the scattering amplitude is violated, it is possible that the σ pole resides on the complex s plane when N_c is large.

The pp→ppx(x=γγ, π⁰π⁰, π⁺π^-) reactions have been measured at CELSIUS in the energy range T_p=775-1360 MeV using the WASA 4π detector with hydrogen pellet target. Close to threshold the ppπ⁺π^- data are in accordance with the Roper resonance concept and/or a dynamic formation of the σ meson. At higher energies the formation of the ΔΔ system becomes increasingly dominant. The ppπ⁰π⁰ channel basically shows similar features, however, develops a peculiar enhancement at small invariant masses M_π⁰π⁰ - in resemblance of Bose-Einstein correlations. Alternatively, the enhancement could be also indicative of a (dynamic) isospin breaking due to ππ rescattering in the σ channel. The latter is favored by the results on pd→³Hex (x=π⁰π⁰, π⁺π^-), where the observed π⁰π⁰ enhancements are still much larger. In the γγ channel a line at M_γγ=2m_π is observed. The possible nature of this line is discussed.

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.