Measurement of Inelastic J/Psi Photoproduction at HERA

TL;DR

This paper reports a measurement of inelastic J/ψ photoproduction cross sections at HERA, comparing experimental data with theoretical models, and finds good agreement with NLO colour-singlet calculations but not with certain colour-octet predictions.

Contribution

It provides the first detailed measurement of inelastic J/ψ photoproduction cross sections at HERA and tests theoretical models against the data.

Findings

NLO colour-singlet model agrees with data in specified kinematic range.

Leading order colour-octet model does not match the experimental results.

Cross sections and distributions are measured after subtracting background processes.

Abstract

We present a measurement of the inelastic, non diffractive J/$\psi$ photoproduction cross section in the reaction $e^{+} p \to e^{+} {J}/\psi X$ with the ZEUS detector at HERA. The J/$\psi$ was identified using both the $\mu^{+}\mu^{-}$ 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/$\psi$ 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/$\psi$ 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/$\psi$ hadroproduction, are not consistent with the data.