Color evaporation and elastic Y photoproduction at DESY HERA

The diffractive photoproduction of vector mesons is usually described by considering the two-gluon ~Pomeron! exchange, the nondiagonal parton distributions, and the contribution of the real part to the cross section. In this Brief Report we analyze the diffractive photoproduction of the Y at DESY HERA using an alternative model, the color evaporation model ~CEM!, where the cross section is simply determined by the boson-gluon cross section and an assumption for the production of the colorless state. We verify that, as in the J/c case, the HERA data for this process can be well described by the CEM. Moreover, we propose the analysis of the ratio R5sY /sJ/c to discriminate between the different approache