Photoproduction of J/$ψ$ in the forward region

Using the NRQCD factorization formalism of Bodwin, Braaten, and Lepage, we calculate the contribution of the subprocess $\gamma g \to c\bar{c}$ to the inclusive forward scattering cross section for $\psi$ production in photon-nucleon collisions. Our theoretical cross section, which depends linearly on the gluon distribution in the nucleon, has energy dependence consistent with that of fixed-target experimental data. The result is proportional to a parameter $\Theta$, defined as a linear combination of NRQCD color-octet matrix elements; through a fit to the data we determine $\Theta = 0.02 \; \mbox{GeV}^{3}$. Extrapolating from the low-energy fixed-target data into the high-energy range of the ZEUS and H1 measurements, we find remarkable agreement between data and theory for the MRS A, MRS A', and CTEQ 3M structure functions, but less so for MRS~G and GRV.