Photoproduction of $\eta_c$ in NRQCD

TL;DR

This paper calculates the photoproduction of eta_c within NRQCD, revealing the dominance of color-octet contributions and suggesting measurements could resolve discrepancies in J/psi production data.

Contribution

It demonstrates that the color-singlet contribution vanishes at multiple orders, highlighting the importance of color-octet processes in eta_c photoproduction.

Findings

Color-singlet contribution vanishes at leading and next-to-leading order.

Dominant contribution from color-octet ${}^1S_0^{(8)}$ subprocess.

Measurement could clarify conflicts in J/psi photoproduction data.

Abstract

We present a calculation for the photoproduction of $\eta_c$ 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 $\eta_c$ is related to that of ${}^3S_1^{(8)}$ of $J/\psi$ by the heavy quark spin symmetry, and the latter can be determined from the direct production of $J/\psi$ 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 $J/\psi$ photoprodution.