Confirmation of the Sigma Meson
A very general model and an analysis of data on the lightest ${0}^{++}$ meson nonet shows that the ${f}_{0}(980)$ and ${f}_{0}(1300)$ resonance poles are two manifestations of the same $s\overline{s}$ state. On the other hand, the $u\overline{u}+d\overline{d}$ state, when unitarized and strongly distorted by hadronic mass shifts, becomes an extremely broad (880 MeV) and light (860 MeV) resonance, with its pole at $s\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.158\ensuremath{-}i0.235{\mathrm{GeV}}^{2}$. This is the $\ensuremath{\sigma}$ meson required by models for spontaneous breaking of chiral symmetry. It has been named the Higgs meson of QCD, because it generates most of the light hadron masses. It dominates $\ensuremath{\pi}\ensuremath{\pi}$ scattering below 900 MeV and it is also the resonance required by nuclear physics.