On the $\eta'$ Gluonic Admixture

The $\eta'$ which is an $SU(3)_F$ singlet state can contain a pure gluon component, gluonium. We examine this possibility by analysing all available experimental data. It is pointed out that the $\eta'$ gluonic component may be as large as 26%. We also show that the amplitude for $J/\psi -> \eta'\gamma$ decay obtains a notable contribution from gluonium.