Measurement of the cross section for the reaction γp → J/ψ p with the ZEUS detector at HERA

Abstract

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