Color-Octet Contributions to $J/ψ$ Photoproduction

TL;DR

It is demonstrated that distinctive color-octet signatures should be visible in {ital J}/{psi} photoproduction and predictions appear at variance with recent experimental data obtained at HERA, indicating that the phenomenological importance of the color- octet contributions is smaller than expected from theoretical considerations and suggested by the Fermilab Tevatron fits.

Abstract

We have calculated the leading color-octet contributions to the production of $J/ψ$ particles in photon-proton collisions. Using the values for the color-octet matrix elements extracted from fits to prompt $J/ψ$ data at the Tevatron, we demonstrate that distinctive color-octet signatures should be visible in $J/ψ$ photoproduction. However, these predictions appear at variance with recent experimental data obtained at HERA, indicating that the phenomenological importance of the color-octet contributions is smaller than expected from theoretical considerations and suggested by the Tevatron fits.

Figures (4)

• Figure 1: Generic diagrams for $J\!/\!\psi$ photoproduction: (a) leading color-singlet contribution; (b) leading color-octet contributions; (c) color-octet contributions to inelastic $J\!/\!\psi$ production.
• Figure 2: Total cross section for $J\!/\!\psi$ photoproduction in the region $z\ge 0.95$ and $p_\perp \le 1$ GeV as a function of the photon-proton energy. Experimental data from H1.
• Figure 3: Total cross section for $J\!/\!\psi$ photoproduction in the region $z\le 0.8$ and $p_\perp \ge 1$ GeV as a function of the photon-proton energy. Experimental data from H1.
• Figure 4: The $J\!/\!\psi$ energy distribution $\hbox{d}\sigma/\hbox{d}{}z$ at the photon-proton centre of mass energy $\sqrt{s\space}\!\!\!\!\! _{\gamma p}\,\, = 100$ GeV integrated in the range $p_\perp \ge 1$ GeV. Experimental data from H1Z.