Associate J / ψ + γ Production : A Clean Probe of Unpolarized and Polarized Gluon Densities

Color-octet contributions to the associate J/ψ+γ are found to be negligible, compared to the ordinary color-singlet contribution. Within the color-singlet model the J/ψ + γ production in the leading order is possible only through gluon-gluon fusion process. Therefore, the associate J/ψ+γ production remains to be useful as a clean channel to probe the unpolarized and polarized gluon distribution inside proton and to study heavy quarkonia production mechanism. We discuss in detail the associate production of J/ψ + γ at pp (or pp) and ep colliders. By requiring the J/ψ to decay into an e + e − or µ + µ − pair, we end up with an exceptionally clean final state. This process can therefore serve as a very clean probe of the gluon densities inside the proton as well as the photon. Numerical results are presented for the TEVATRON pp and HERA ep colliders. This same mechanism can be used to probe the polarized gluon content of the proton in polarized p + p(¯ p) collisions. We study in detail J/ψ + γ production at both polarized fixed target and polarized collider energies for RHIC.