Abstract
The scalar glueball is observed in a coupled-channel analysis of the S-wave amplitude from BESIII data on radiative J/ψ decays and further data. Ten scalar isoscalar resonances were required to fit the data. Five of them were interpreted as mainly-singlet, five as mainlyoctet resonances in SU(3). The yield of resonances showed a striking peak with properties expected from a scalar glueball:
• G0(1865) is produced abundantly in radiative J/ψ decays above a very low background. Its mass is 1σ compatible with the mass calculated in unquenched lattice QCD, and the yield is 1.6σ compatible with the yield calculated in lattice QCD.
• The decay analysis of the scalar isoscalar mesons shows that the assignment of mesons to mainly-octet and mainly-singlet states is correct. Even the production of mainly-octet scalar mesons - which should be forbidden in radiative J/ψ decays - peaks at 1865 MeV. The decay analysis requires a small glueball content in the flavor wave function of several scalar resonances. The glueball content as a function of the mass shows a peak compatible with the peak in the yield of scalar isoscalar mesons. The sum of the fractional glueball contributions is compatible with one.
• In the reaction Bs → J/ψ + K+K− reported by the LHCb collaboration, a primary ss̄ couples to mesons having a strong coupling to K+K−. Two peaks in the K+K− mass spectrum are seen due to ϕ(1020) and f′2(1525), but there is little evidence for the f0(1710) or other high-mass scalar mesons coupling strongly to KK̄. High-mass scalar mesons are strongly produced by two initial-state gluons but not by an ss̄ pair in the initial state. They must have sizable glueball fractions!
• The D wave amplitude in the BESIII data on radiative J/ψ decays reveales a high-mass structure which can be described by a single Breit-Wigner or by the sum of three ϕϕ resonances interpreted as tensor glueballs a long time ago. The structure - and further tensor resonances observed in radiative J/ψ decays - are tentatively interpreted as tensor glueball.
• In J/ψ decays into γπ0π0η′ several resonances are reported. The possibility is discussed that the pseudoscalar glueball might be hidden in these data.