The light scalar mesons within quark models
  Low-energy meson-meson scattering data are a powerful testing ground for quark models. Here, we describe the behaviour at threshold of S-wave scattering-matrix singularities.   The majority of the full scattering-matrix mesonic poles stem from an underlying confinement spectrum. However, the light scalar mesons K0*(830), a0(980), f0(400-1200), and f0(980) do not, but instead originate in 3P0-barrier semi-bound states. We show that the behaviour of the corresponding poles is identical at threshold.   In passing, the light-meson sector is given a firm basis. 
Eef van Beveren, George Rupp, Nicholas Petropoulos and Frieder   Kleefeld