Significance of the Sigma Meson in Hadron Physics (QCD) and Possible Experiments to Observe it

Abstract

We first discuss the theoretical and phenomenological significance of the sigma meson ($\sigma$) in QCD. It is indicated that if the collective modes with the mass 500-600 MeV exists in the $I=J=0$ channel, various empirical facts in hadron physics can be naturally accounted for, which otherwise would remain mysterious. We propose several experiments to produce and detect the $\sigma$ in nuclei using nuclear and electro-magnetic projectiles. The recent CHAOS data which show a spectral enhancement near the 2 $m_{\pi}$ threshold in the $\sigma$ channel from the reactions A$(\pi, 2\pi)$A' where A and A' denotes nuclei is interpreted as a possible evidence of a partial restoration of chiral symmetry in nuclei.