Colour-octet contributions to J/ψ photoproduction via fragmentation at HERA

We study J/psi photoproduction via fragmentation at next-to-leading order in the QCD-improved parton model, using the nonrelativistic factorization formalism proposed by Bodwin, Braaten, and Lepage. We consider direct and resolved photoproduction of prompt J/psi mesons and chi(cJ) mesons radiatively decaying to J/psi + gamma, taking into account the formation of both colour-singlet and colour-octet c (c) over bar states. Adopting the values of the long-distance colour-octet matrix elements extracted from fits to prompt-J/psi data recently taken at the Fermilab Tevatron, we predict that measurements of J/psi photoproduction at DESY HERA should show a distinctive excess over the expectation based on the colour-singlet model at small values of the inelasticity variable z. This is complementary to the expected enhancement at z less than or similar to 1 due to the colour-octet contribution to photon-gluon fusion. (C) 1997 Elsevier Science B.V.