Exclusive photoproduction of a heavy vector meson in QCD

TL;DR

This paper develops a one-loop, collinear-factorization framework for exclusive heavy vector meson photoproduction gamma p -> V p, combining NRQCD with generalized parton distributions to factorize the amplitude into a perturbative hard kernel and nonperturbative inputs. Using dispersion relations and a soft-gluon emission theorem, the authors derive finite NLO hard-scattering kernels T_q and T_g, analyze their high-energy behavior, and confront predictions with HERA data using GPD models. Their numerical study shows that NLO corrections significantly reduce scale uncertainty for Υ but are large and potentially uncontrolled for J/ψ at current energies, highlighting the need for small-x resummation. Overall, the work demonstrates a consistent factorization picture at one loop and points to resummation as a key ingredient for reliable high-energy predictions in heavy-quarkonium photoproduction.

Abstract

The process of exclusive heavy vector meson photoproduction, gamma p -> V p, is studied in the framework of QCD factorization. The mass of the produced meson, V = Upsilon or J/Psi, provides a hard scale for the process. We demonstrate, that in the heavy quark limit and at the one-loop order in perturbation theory, the amplitude factorizes in a convolution of a perturbatively calculable hard-scattering amplitude with the generalized parton densities and the nonrelativistic QCD matrix element <O_1>_V. We evaluate the hard scattering amplitude at one-loop order and compare the data with theoretical predictions using an available model for generalized parton distributions.

Figures (17)

• Figure 1: Kinematics of heavy vector meson photoproduction.
• Figure 2: The hard-scattering amplitude at LO.
• Figure 3: The light quark contribution to heavy meson photoproduction.
• Figure 4: The analytical properties of the partonic amplitudes at NLO in the complex plane of $y=x_2/\zeta$.
• Figure 5: The $\tilde{s}$- channel cut diagrams for the quark amplitude.