Charmonium Production via Fragmentation at DESY HERA
Bernd A. Kniehl, Gustav Kramer
TL;DR
This study investigates charmonium production at large transverse momentum in photon-proton interactions at HERA by treating charm fragmentation within a massless-charm factorization framework and NRQCD. It provides NLO fragmentation cross sections for $J/\psi$ production in both direct and resolved photoproduction channels, including colour-singlet and colour-octet contributions, and accounts for feeddown from $\chi_{cJ}$ decays. The analysis shows fragmentation, particularly colour-octet fragmentation, can dominate over the conventional parton-fusion mechanism at large $p_T$, with resolved-photon processes playing a crucial role at lower $p_T$ and forward rapidities. The results suggest that forward, high-$W$ kinematics at HERA are optimal for testing NRQCD colour-octet fragmentation and its connection to Tevatron observations of anomalously large $J/\psi$ production.
Abstract
The cross section for the photoproduction of large-p_T J/psi 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/psi 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 colour-octet components of the J/psi wave function, which contribute to the fragmentation process, is emphasized. In addition to prompt J/psi production, we consider also the production of chi_{cJ} states followed by radiative decays to J/psi mesons, both in the colour-singlet and colour-octet channels.