Inelastic Photoproduction at HERA: a Second Charmonium Crisis?

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.