Charmonium Production via Fragmentation at DESY HERA

The cross section for the photoproduction of large-pT J/ψ mesons at HERA is calculated at next-to-leading order, adopting a perturbative approach to describe the fragmentation of charm quarks and gluons into J/ψ mesons. We treat the charm quark according to the massless factorization scheme, where it is assumed to be one of the active flavours inside the proton and the resolved photon. We present inclusive distributions in transverse momentum and rapidity, including the contributions due to direct and resolved photons. The importance of the colouroctet components of the J/ψ wave function, which contribute to the fragmentation process, is emphasized. In addition to prompt J/ψ production, we consider also the production of χcJ states followed by radiative decays to J/ψ mesons, both in the colour-singlet and colour-octet channels. PACS numbers: 13.60.-r, 13.85.Ni, 13.87.Fh, 14.40.Lb ∗Supported by Bundesministerium für Forschung und Technologie, Bonn, Germany, under Contract 05 7 HH 92P (5), and by EU Program Human Capital and Mobility through Network Physics at High Energy Colliders under Contract CHRX–CT93–0357 (DG12 COMA).