Systematics of quark-antiquark states and scalar exotic mesons

Analysis of experimental data from the Crystal Barrel Collaboration on the in-flight $p\bar p$ annihilation into mesons revealed a large group of mesons in the 1900–2400 MeV range, allowing the systematization of quark-antiquark states in the $(n,M^2)$- and $(J,M^2)$-planes ($n$ being the radial quantum number, and $J$ the spin of a meson of mass $M$). From the data, meson trajectories in these planes are approximately linear and have a universal slope. A detailed discussion of the scalar meson sector is given for which, based on a recent $K$-matrix analysis, the nonet classification of quark–antiquark states places two scalar nonets — the basic nonet and that of the first radial excitation — in the range below 2000 MeV. Two isospin 0 scalar states — the broad resonance $f_0(1200-1600)$ and the light $\sigma$-meson — do not fit into the quark-antiquark classification, i.e., are exotic. The ratios between the coupling constants for hadronic decays to the $\pi\pi$, $K\bar K$, $\eta\eta$ and $\eta\eta'$ states point to the gluonium nature of the broad state $f_0(1200-1600)$.