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

Abstract

This paper reports the cross section measurements for the process ep $\to$ 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 ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ and ${\mu }^{+}{\mu }^{-}$ decay modes. The photoproduction cross section was measured to be $52_{-12}^{+7}\pm 10$~nb at an average $\gamma$p centre of mass energy of 67 GeV and $71_{-20}^{+13}\pm 12$~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.

21 pages, latex, 4 figures appended as uuencoded file

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