The lightest scalar glueball

Recent studies of meson spectra have enabled the resonance structure of the $IJ^{\mathrm{PC}}=00^{++},10^{++},02^{++},12^{++}$, and $IJ^{\mathrm P}=\frac120^+$ waves to be found for masses ranging up to $1900$ MeV, thus fully reconstructing the $1^3\mathrm P_0\mathrm q\overline{\mathrm q}$ and $2^3\mathrm P_0\mathrm q\overline{\mathrm q}$ meson multiplets. There is firm experimental evidence for the existence of five scalar–isoscalar states in this mass range, four of which are $\mathrm q\overline{\mathrm q}$-states and members of the $1^3\mathrm P_0\mathrm q\overline{\mathrm q}$ and $2^3\mathrm P_0\mathrm q\overline{\mathrm q}$ nonets, whereas the fifth falls out of the quark picture and displays all the properties of the lightest possible scalar glueball. A dispersion analysis of the $00^{++}$ wave elucidates how the mixture of the pure glueball state (or gluonium) with neighboring scalar $\mathrm q\overline{\mathrm q}$-states forms: three scalar mesons, namely two relatively narrow$\mathrm f_0(1300)$ and $\mathrm f_0(1500)$ resonances and a very broad $\mathrm f_0(1530_{-250}^{+90})$ resonance, share the gluonium, the broad resonance being the gluonium's descendant and accounting for about $40$ to $50\%$ of it.