X₂ production in polarized pp collisions at RHIC: Measuring ΔG and testing the color octet model

We consider the production and decay of the X₂ charmonium state in polarized and unpolarized pp collisions at RHIC in the framework of an effective theory based on the QCD multipole expansion. We find that the angular distribution in the decay of the produced charmonium, X₂ → J/ψ + γ, in the unpolarized case allows us to distinguish clearly between the color singlet and color octet production mechanisms. Once the production mechanism is known, the angular distribution in the polarized case can be used to measure the polarized gluon distribution in the proton, ΔG(x).