Systematics of quark-antiquark states, and exotic scalar mesons

Analysis of experimental data from the Crystal Barrel Collaboration on pp-bar-annihilation in flight into mesons revealed a large group of mesons in the 1900-2400 MeV range, allowing the systematization of quark-antiquark states in the (n, M2)- and (J, M2)- 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 f0(1200-1600) and the light σ-meson - do not fit into the quark-antiquark classification, i.e., are exotic. The ratios between the coupling constants for hadronic decays to the ππ, KK-bar, ηη, and ηη' states point to the gluonium nature of the broad state f0(1200-1600)