Glueball nature of the sigma/f_0(600) from pi-pi and gamma-gamma   scatterings
  We estimate the I=0 scalar meson sigma/f_0(600) parameters from pi-pi and gamma-gamma scattering data below 700 MeV using an improved analytic K-matrix model. A fit of the hadronic data gives a complex pole mass M_sigma= 422-i290 MeV, while simultaneous best fits of the gamma-gamma to pi+pi-, pi0pi0 data give a direct width of (0.13+-0.05)keV, a rescattering component of (2.7+-0.4)keV and a total (direct+rescattering) width of (3.9+-0.6)keV. "Running" these results to the physical real axis, the small "direct" gamma-gamma and the large hadronic widths at the "on-shell" mass are compatible with QCD spectral sum rules (QSSR) and some low-energy theorems (LET) expectations for an unmixed lowest mass glueball/gluonium sigma_B of a mass around 1 GeV and a large OZI-violation decay into pi-pi. 
G. Mennessier, S. Narison and W. Ochs