The Fragmented Glueball: A Personal View

A coupled-channel analysis has been performed to identify the spectrum of scalar mesons. The data include BESIII data on radiative $J/ψ$ decays into $π^0π^0$,$K_SK_S$, $ηη$, and $ωϕ$, 15 Dalitz plots from $\bar pN$ annihilation at rest at LEAR, the CERN-Munich multipoles for $ππ$ elastic scattering, the $S$-wave from BNL data on $ππ$ scattering into $K_SK_S$, from GAMS data on $ππ\to π^0π^0, ηη$, and $ηη'$, and NA48/2 data on low-mass $ππ$ interactions from $K^\pm\toππe^\pmν$ decays. The analysis reveals the existence of ten scalar isoscalar resonances. The resonances can be grouped into two classes: resonances with a large SU(3) singlet component and those with a large octet component. The production of isoscalar resonances with a large octet component should be suppressed in radiative $J/ψ$ decays. However, in a limited mass range centered at 1900\,MeV, these mesons are produced abundantly. Mainly-singlet scalar resonances are produced over the full mass range but with larger intensity at 1900\,MeV. The total scalar isoscalar yield in radiative decays into scalar mesons shows a clear peak which is interpreted as the scalar glueball of lowest mass.

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