Hard exclusive $J/\psi$ leptoproduction on polarized targets

Abstract

We consider the exclusive production of $J/\psi$ mesons in polarized virtual-photon--proton collisions. We derive helicity amplitudes for the dominant subprocess ${\gamma }^{\ast }g\to J/\psi g$ and show that polarization asymmetries vanish in the case of collinear scattering. Thus, contrary to what has been suggested earlier, this process is not a good probe of the polarized gluon distribution $\Delta g(x,Q^2)$ of the proton.

Comments: Latex, 12 pages, 1 ps figure; typos corrected, some points clarified in the text