Inelastic electroproduction of $\eta_c$ at ep colliders
  Using the nonrelativistic QCD factorization formalism, we calculate the electroproduction cross sections of $\eta_c$ in ep collisions, including the contribution from both the transverse photon and the longitudinal photon. For this process the color-singlet contribution vanishes up to the next to leading order perturbative QCD calculations. The dominant contribution comes from the color-octet ${}^1S_0^{(8)}$ subprocess. The nonperturbative color-octet matrix element of ${}^1S_0^{(8)}$ of $\eta_c$ is related to that of ${}^3S_1^{(8)}$ of $J/\psi$ by heavy quark spin symmetry, and the latter can be determined from the direct production of $J/\psi$ at large transverse momentum at the Fermilab Tevatron. The measurement of this process at DESY HERA can be viewed as another independent test for the color-octet production mechanism 
Li-Kun Hao, Feng Yuan, and Kuang-Ta Chao