Ba. Kniehl et G. Kramer, COLOR-OCTET CONTRIBUTIONS TO J PSI PHOTOPRODUCTION VIA FRAGMENTATION AT HERA/, Physics letters. Section B, 413(3-4), 1997, pp. 416-421
We study J/psi photoproduction via fragmentation at next-to-leading or
der in the QCD-improved parton model, using the nonrelativistic factor
ization 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 th
e formation of both colour-singlet and colour-octet c (c) over bar sta
tes. Adopting the values of the long-distance colour-octet matrix elem
ents extracted from fits to prompt-J/psi data recently taken at the Fe
rmilab Tevatron, we predict that measurements of J/psi photoproduction
at DESY HERA should show a distinctive excess over the expectation ba
sed on the colour-singlet model at small values of the inelasticity va
riable z. This is complementary to the expected enhancement at z less
than or similar to 1 due to the colour-octet contribution to photon-gl
uon fusion. (C) 1997 Elsevier Science B.V.