Color Evaporation and Elastic $\Upsilon$ Photoproduction at DESY HERA

Abstract

The diffractive photoproduction of vector mesons is usually described considering the two-gluon (Pomeron) exchange, non-diagonal parton distributions and the contribution of the real part to the cross section. In this letter we analyze the diffractive photoproduction of the $\Upsilon$ at 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, similarly to the $J/\Psi$ case, the HERA data for this process can be well described by the CEM. Moreover, we propose the analyzes of the ratio $R = \sigma_{\Upsilon} / \sigma_{J/\Psi}$ to discriminate between the distinct approaches.