Scalar meson spectroscopy: achievements and traps
  Interactions in three coupled channels: pi-pi, K-anti K and sigma-sigma have been investigated in a wide two-pion effective mass region from the pi-pi threshold up to 1600 MeV. Analytical structure of amplitudes in all channels has been studied. It was shown that its knowledge is necessary to understand spectrum of scalar mesons and their nature. 
Robert Kaminski, Leonard Lesniak and Benoit Loiseau