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Measurement of Inelastic J/Psi Photoproduction at HERA

We present a measurement of the inelastic, non diffractive J/$ψ$ photoproduction cross section in the reaction $e^{+} p \to e^{+} {J}/ψX$ with the ZEUS detector at HERA. The J/$ψ$ was identified using both the $μ^{+}μ^{-}$ and $e^{+}e^{-}$ decay channels and events were selected within the range $0.4<z<0.9$ ($0.5<z<0.9$) for the muon (electron) decay mode, where $z$ is the fraction of the photon energy carried by the J/$ψ$ in the proton rest frame. The cross section, the $p^2_T$ and the $z$ distributions, after having subtracted the contributions from resolved photon and diffractive proton dissociative processes, are given for the photon-proton centre of mass energy range $50<W<180$ GeV; $p^2_T$ is the square of the J/$ψ$ transverse momentum with respect to the incoming proton beam direction. In the kinematic range $0.4 < z < 0.9$ and $p^2_T > 1$ GeV$^2$, NLO calculations of the photon-gluon fusion process based on the colour-singlet model are in good agreement with the data. The predictions of a specific leading order colour-octet model, as formulated to describe the CDF data on J/$ψ$ hadroproduction, are not consistent with the data.