Photoproduction of J/$ψ$ in the forward region

TL;DR

This work tests the NRQCD color-octet mechanism in forward J/psi photoproduction by computing the leading color-singlet and forward-octet contributions and extracting the forward-octet combination Θ from fixed-target elastic data. The estimated Θ ≈ 0.02 GeV^3 aligns with HERA measurements and other determinations within large theoretical uncertainties, supporting NRQCD scaling. The paper discusses conflicts with diffractive approaches and emphasizes polarization and differential cross sections as key diagnostics for distinguishing production mechanisms.

Abstract

We study the phenomenology of fixed-target elastic $J/ψ$ photoproduction in the NRQCD factorization formalism. Our the goal is to test an essential feature of this formalism --- the color-octet mechanism. We obtain an order-of-magnitude estimate for a certain linear combination of NRQCD color-octet matrix elements. Our estimate is consistent with other empirical determinations and with the $v$-scaling rules of NRQCD.

Figures (4)

• Figure 1: Leading order diagrams for the photoproduction of a $c\bar{c}$ in a color-singlet, ${}^3S_1$ state.
• Figure 2: Leading order diagrams for the photoproduction of a $c\bar{c}$ in a color-octet state with angular momentum configuration ${}^1S_0$, ${}^3P_0$, and ${}^3P_2$. These diagrams are the underlying subprocess for the forward octet contributions to $J/\psi$ photoproduction.
• Figure 3: Fit to fixed-target experiments. Shown are data points for the elastic photoproduction cross section measured at fixed-target experiments. Also shown are the theoretical curves evaluated numerically using five different gluon distribution functions. In the curve $\Theta$ is adjusted to best fit the data.
• Figure 4: Comparison of NRQCD forward octet theoretical prediction to HERA data using the value of $\Theta$ determined from fixed target experiments. Shown are data points for the elastic scattering cross section measured at various HERA and Fermilab experiments.