Photoproduction of $η_c$ in NRQCD

Abstract

We present a calculation for the photoproduction of $η_c$ under the framework of NRQCD factorization formalism. We find a quite unique feature that the color-singlet contribution to this process vanishes at not only the leading order but also the next to leading order perturbative QCD calculations and that the dominant contribution comes from the color-octet ${}^1S_0^{(8)}$ subprocess. The nonperturbative color-octet matrix element of ${}^1S_0^{(8)}$ of $η_c$ is related to that of ${}^3S_1^{(8)}$ of $J/ψ$ by the heavy quark spin symmetry, and the latter can be determined from the direct production of $J/ψ$ at large transverse momentum at the Fermilib Tevatron. We then conclude that the measurement of this process may clarify the existing conflict between the color-octet prediction and the experimental result on the $J/ψ$ photoprodution.