Measurement of the Cross Section for the Reaction $\gamma p \to \jpsi~p$ with the ZEUS Detector at HERA

Abstract

This paper reports the cross section measurements for the process ep $\rightarrow$ e~\jpsi~p for $Q^2 < 4$ GeV$^2$ at $\sqrt s = 296$ GeV, based on an integrated luminosity of about 0.5~pb$^{-1}$, using the ZEUS detector. The \jpsi~was detected in its $\rm e^{+}e^{-}$ and $\rm \mu^{+}\mu^{-}$ decay modes. The photoproduction cross section was measured to be $52^{ \ +7}_{-12}\pm 10$~nb at an average $\gamma$p centre of mass energy of 67 GeV and $71^{+13}_{-20}\pm 12$~nb at 114 GeV. The significant rise of the cross section compared to lower energy measurements is not in agreement with VDM models, but can be described by QCD inspired models if a rise in the gluon momentum density at low $x$ in the proton is assumed.