Gluonia in QCD with massless quarks

I briefly review the estimate of the gluonia masses, decay and mixings in QCD with massless quarks from QCD spectral sum rules and some low-energy theorems. The data in the $0^{++}$ channel can be explained with some maximal gluonium-quarkonium mixing schemes, which then suggest a large violation of the OZI rule, similar to the case of the ${\eta }^{\mathrm{\prime }}$ in the $U(1{)}_A$ sector.