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

Abstract

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 color-singlet and color-octet c anti-c states. Adopting the values of the long-distance color-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 color-singlet model at small values of the inelasticity variable z. This is complementary to the expected enhancement at z close to 1 due to the color-octet contribution to photon-gluon fusion.

Figures (3)

• Figure 1: Cross section $d\sigma/dz$ of inelastic $J/\psi$ photoproduction at HERA, integrated over 60 GeV${}<W<280$ GeV and 4 GeV${}<p_T<12$ GeV (upper curves) or 8 GeV${}<p_T<12$ GeV (lower curves). The NLO fragmentation contributions due to direct photons (dotted lines), resolved photons (dot-dashed lines), and their sum (solid lines) are compared with the LO $\gamma g$-fusion contribution (dashed lines).
• Figure 2: Cross section $d\sigma/dp_T$ of inelastic $J/\psi$ photoproduction at HERA, integrated over 60 GeV${}<W<280$ GeV and $0.05<z<0.8$ (upper curves) or $0.05<z<0.4$ (lower curves). The total NLO fragmentation contribution (solid lines) is compared with the LO $\gamma g$-fusion contribution (dashed lines). For comparison, also the prompt color-singlet (dotted lines) and the non-prompt (dot-dashed lines) fragmentation contributions are shown.
• Figure 3: Cross section $d\sigma/dp_T^2$ of inelastic $J/\psi$ photoproduction at HERA, integrated over 60 GeV${}<W<130$ GeV and $0.3<z<0.8$ (lower curves) and divided by the corresponding photon flux factor, $6.66\times10^{-2}$. The total NLO fragmentation (solid lines) and LO $\gamma g$-fusion (dashed lines) contributions are compared with the ZEUS data col. For comparison, the theoretical results are also shown for $0.05<z<0.8$ (upper curves).