Forward Cone in Exclusive Heavy Vector Meson Production

Abstract

The color dipole gBFKL phenomenology of a diffraction cone for photo- and electroproduction ${\gamma }^{\ast }N\to VN$ of heavy vector mesons (charmonium & bottonium) at HERA and in fixed target experiments is presented. A substantial shrinkage of the diffraction cone from the CERN/FNAL to the HERA range of c.m.s. energy $W$ is predicted. The $Q^2$-controlled selectivity to the color dipole size (scanning phenomenon) is shown to lead to a decrease of the diffraction slope with $Q^2$ (which is supported by the available experimental data). An approximate flavor independence of the diffraction slope in the scaling variable $Q^2+m_V^2$ is shown to hold.

6 pages, Latex