Colour-octet contributions to JΨ photoproduction via fragmentation at HERA

We study JΨ 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 JΨ mesons and χcJ mesons radiatively decaying to JΨ + γ, taking into account the formation of both colour-singlet and colour-octet cc states. Adopting the values of the long-distance colour-octet matrix elements extracted from fits to prompt-J/Ψ data recently taken at the Fermilab Tevatron, we predict that measurements of JΨ 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 ≲ 1 due to the colour-octet contribution to photon-gluon fusion.