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

TL;DR

This paper measures the cross section of the process gamma p to J/psi p at HERA, showing a rise with energy that challenges traditional models but aligns with QCD predictions involving gluon density growth.

Contribution

First measurement of the gamma p to J/psi p cross section at HERA energies using ZEUS detector data, providing insights into gluon distribution in the proton.

Findings

Cross section increases with energy, contradicting VDM models.

Results support QCD models with rising gluon density at low x.

Measured cross sections at 67 GeV and 114 GeV center of mass energy.

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.