Radiative decay of vector quarkonium: Constraints on glueballs and light gluinos

Given a resonance of known mass, width, and ${\mathit{J}}^{\mathit{P}\mathit{C}}$, we can determine its gluonic branching fraction ${\mathit{b}}_{\mathit{R}\to \mathit{g}\mathit{g}}$ from data on it s production in radiative vector quarkonium decay V→γR. For most resonances ${\mathit{b}}_{\mathit{R}\to \mathit{g}\mathit{g}}$ is found to be ∼10%, consistent with being qq¯ states, but we find that both pseudoscalars observed in the 1440 MeV region have ${\mathit{b}}_{\mathit{R}\to \mathit{g}\mathit{g}}$∼1/2-1, and b(${\mathit{f}}_0^{++}$→gg)∼1/2. As data improve, ${\mathit{b}}_{\mathit{R}\to \mathit{g}\mathit{g}}$ should be a useful discriminator between qq¯ and gluonic states and may permit quantitative determination of the extent to which a particular resonance is a mixture of glueball and qq¯. We also examine the regime of validity of PQCD for predicting the rate of V→γ${\mathrm{\eta }}_{\mathit{g}\mathrm{̃}}$, the ‘‘extra’’ pseudoscalar bound state which would exist if there were light gluinos. From the CUSB limit on peaks in Υ→γX, the mass range 3 GeV≲m(${\mathrm{\eta }}_{\mathit{g}\mathrm{̃}}$)≲7 GeV can be excluded. An experiment must be significantly more sensitive to exclude an ${\mathrm{\eta }}_{\mathit{g}\mathrm{̃}}$ lighter than this.