Hard exclusive $J\mathrm{/}\psi$ leptoproduction on polarized targets

We consider the exclusive production of $J\mathrm{/}\psi$ mesons in polarized virtual-photon–proton collisions. We derive helicity amplitudes for the dominant subprocess ${\gamma }^{\ast }g\to J\mathrm{/}\psi g$ 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 $\mathrm{\Delta }g(x,Q^2)$ of the proton.