Heavy Quarkonium Photoproduction at HERA and Charm Content of the Photon

A study of the J/ψ-meson photoproduction at large transverse momentum at HERA energies via the charm quark excitation in the photon is presented. Based on perturbative QCD, nonrelativistic quark model and photon structure function formalism, our calculation demonstrates that the charm content of the photon may be very important for the J/ψ photoproduction at the large transverse momentum. It is shown that at HERA energies J/ψ production via the subprocess c(γ)g → J/ψc dominates over the resolved photon contribution via the photon-gluon fusion at p⊥ ≥ 2 GeV/c and over the leading order direct photon-gluon fusion contribution at p⊥ ≥ 15 GeV/c. The study of a different mechanisms of the inclusive J/ψ photoproduction on protons is of a particular importance because the process of heavy quarkonium production via photon-gluon fusion plays a crucial role in the measurement of gluon structure function in a proton [1, 2, 3, 4]. Previous calculations of J/ψ photoproduction at large p⊥ have included direct charmonium production via photon-gluon fusion using the colour singlet model in LO [1, 2] and NLO approximation [5], diffractive inelastic J/ψ production [6], resolved photon contributions via subprocesses gg → J/ψg, gg → χc → J/ψγ [4] and J/ψ production from b quark decays [7]. Here we examine the diffractive-like contribution of the charm quark excitation in a photon to the J/ψ photoproduction at large p⊥ via partonic subprocess c(γ)g → J/ψc, where charm quarks in the initial state are generated by QCD evolution of the photon structure functions (PSF) [8]. We suppose that at low Q the charm content of the photon is virtually nil, but at Q of order m2c one has sufficient resolution to find charm quarks in photon. Note, that in the processes of the J/ψ photoproduction at large p⊥ the relevant QCD scale Q 2 ∼ M ψ + p2⊥ >> m2c . Our calculation is partly the same as the approach used in Ref.[9] for description of the charm quark hadroproduction and based on the fact that both the central and diffractive components of charm production can be understood in the intrinsic charm structure function approach. Recently we have presented the results of the calculations for contributions of the intrinsic charm in the proton to the J/ψ photoproduction [10] and hadroproduction [11]. The study of a photon structure function [8] in the resolved-photon interaction at HERA energy and beyond is also very interested [12]. Usually it is supposed that at high energy in the resolved-photon interaction the gluon content of a photon is dominant. Howere, the heavy quark content of the photon is also an increasingly important subject of study [13]. In this paper we would like to show that in the J/ψ photoproduction at lager p⊥ via resolved-photon interaction the charm content of a photon is more important and it is may be studied experimentally using J/ψ plus open charm associated photoproduction. The process of the J/ψ photoproduction on a proton in the resolved-photon interaction is schematically presented in Fig. 1.