Inelastic J== Photoproduction

Inelastic photoproduction of J/ψ particles at high energies is one of the processes to determine the gluon distribution in the nucleon. We have calculated the QCD radiative corrections to the color-singlet model of this reaction. They are large at moderate photon energies, but decrease with increasing energies. The cross section and the J/ψ energy spectrum are compared with the available fixed-target photoproduction data and predictions are given for the HERA energy range. ∗ Supported in part by Deutsche Forschungsgemeinschaft DFG † Present address: Deutsches Elektronen-Synchrotron DESY, D-22603 Hamburg FRG; E-mail: [email protected] The measurement of the gluon distribution in the nucleon is one of the important goals of lepton-nucleon scattering experiments. The classical methods exploit the evolution of the nucleon structure functions with the momentum transfer, and the size of the longitudinal structure function. With rising energies, however, jet physics and the production of heavy quark states become important complementary tools. Besides open charm and bottom production, the formation of J/ψ bound states [1] in inelastic photoproduction experiments γ +N → J /ψ + X (1) provides an experimentally attractive method since J/ψ particles are easy to tag in the leptonic decay modes. Many channels contribute to the generation of J/ψ particles in photoproduction experiments [2], similarly to hadroproduction experiments. However, no satisfactory quantitative picture has emerged yet and the production of a large surplus of ψ particles in pp collisions awaits the proper understanding. Theoretical interest so far has focussed on two mechanisms for J/ψ photoand electroproduction, elastic/diffractive [3,4] and inelastic production through photon-gluon-fusion [1,2]. While by the first mechanism one expects to shed light on the physical nature of the pomeron, inelastic J/ψ production provides information on the distribution of gluons in the nucleon [5]. The two mechanisms can be separated by measuring the J/ψ energy spectrum, described by the scaling variable