Color-Octet Contributions to J/ψ Photoproduction via Fragmentation at HERA

We study J/ψ 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/ψ mesons and χcJ mesons radiatively decaying to J/ψ+ γ, taking into account the formation of both color-singlet and color-octet cc̄ states. Adopting the values of the long-distance color-octet matrix elements extracted from fits to prompt-J/ψ data recently taken at the Fermilab Tevatron, we predict that measurements of J/ψ photoproduction at DESY HERA should show a distinctive excess over the expectation based on the color-singlet model at small values of the inelasticity variable z. This is complementary to the expected enhancement at z∼< 1 due to the color-octet contribution to photon-gluon fusion. 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 EEC Program Human Capital and Mobility through Network Physics at High Energy Colliders under Contract CHRX–CT93–0357 (DG12 COMA).