Mesons of the $f_0$ family in processes $\pi\pi\to\pi\pi,K\bar{K}$ up to   2 GeV and chiral symmetry
  In a combined analysis of the experimental data on the coupled processes $\pi\pi\to\pi\pi,K\bar{K}$ in the channel with $I^GJ^{PC}=0^+0^{++}$, the various scenarios of these reactions (with different numbers of resonances) are considered. In a model-independent approach, based only on analyticity and unitarity, a resonance is represented by of a pole cluster (poles on the Riemann surface) of the definite type that is defined by the state nature. The best scenario contains the resonances $f_0(665)$ (with properties of the $\sigma$-meson), ${f_0}(980)$ (with a dominant $s{\bar s}$ component), $f_0(1500)$ (with a dominant flavour-singlet, {\it e.g.}, glueball component) and the ${f_0}(1710)$ (with a considerable $s{\bar s}$ component). If the ${f_0}(1370)$ exists, it has a dominant $s{\bar s}$ component. The coupling constants of observed states with the considered channels and the $\pi\pi$ and $K\bar{K}$ scattering lengths are obtained. The conclusion on the linear realization of chiral symmetry is drawn. 
Yu.S. Surovtsev, D. Krupa, and M. Nagy