Light Scalar Isoscalar Mesons and the Glueball

Five scalar isoscalar (f0) mesons below 1.8 GeV/c2 are experimentally established according to the Review of Particle Physics 2004. Five f0 ground states are theoretically expected, below 1.8 GeV/c2: two qq̄, two q2q̄2 and a glueball. There is no safe prediction how these states mix. Two of the 5 observed f0 resonances are very broad and their mass, width and decay branching ratios have large uncertainties; we cannot even exclude they belong to a single superwide resonance. The glueball can be mixed into any of the 5 or 4 observed f0 mesons. Although the f0 sector is at the most difficult edge of meson spectroscopy, there is hope to obtain consistent experimental results allowing to determine the composition of the observed mesons if experimenters agree on a suitable formalism to extract resonance parameters in this complex environment.