Inelastic Photoproduction at HERA: a Second Charmonium Crisis?
O. J. P. Eboli, E. M. Gregores, F. Halzen
TL;DR
The paper addresses the puzzling results of inelastic J/psi photoproduction at HERA, where the color singlet model (CSM) fails to describe hadroproduction and NRQCD faces difficulties with the HERA data. It proposes the color evaporation mechanism (CEM) as a unified framework that allows both color-singlet and color-octet ccbar states to form quarkonia with a universal nonperturbative probability and tests it against HERA kinematics, including direct and resolved photon processes. With rho_psi about 0.43 and GRV parton densities, the CEM calculations reproduce the energy and z-dependence of the J/psi photoproduction cross section across the measured z range, though higher-order corrections introduce uncertainties. The results argue that the observed data favor the color evaporation description over a pure color-singlet picture and provide testable low-z predictions due to resolved-photon dynamics.
Abstract
The measurement of the inelastic photoproduction of charmonium at HERA seems to have ignited a new charmonium crisis. The (already discredited) color singlet model fits the data for large charmonium energy fraction z, where the NRQCD model qualitatively fails. We here point out that by the straightforward inclusion of color singlet and octet processes in the soft color (color evaporation) scheme, the HERA data can be accommodated for all z. We anticipate that the color singlet model will fail at low z, as it does in hadroproduction.