Measurements of inelastic J/psi and psi' photoproduction at HERA

TL;DR

This ZEUS analysis provides a comprehensive set of measurements for inelastic J/psi and psi' photoproduction at HERA, capturing differential cross sections across z, pT, W, and y and extracting J/psi helicity information. By comparing to LO and NLO colour-singlet QCD calculations and to NRQCD-based CS+CO predictions, the study tests the validity and universality of colour-octet contributions in photoproduction. The NLO CS direct-photon description matches the observed shapes and distributions within large theoretical uncertainties, while CO effects remain difficult to constrain; the psi' to J/psi cross-section ratio is consistent with LO CS expectations and cascades from psi' modestly increase the J/psi yield. Overall, the results provide important benchmarks for NRQCD and highlight the need for improved higher-order calculations and CO matrix-element determinations in quarkonium photoproduction.

Abstract

The cross sections for inelastic photoproduction of J/psi and psi' mesons have been measured in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 38.0 pb-1. The events were required to have 0.1 < z < 0.9 and 50 < W < 180 GeV, where z is the fraction of the incident photon energy carried by the J/psi in the proton rest frame and W is the photon-proton centre-of-mass energy. The psi' to J/psi cross-section ratio was measured in the range 0.55 < z < 0.9. The J/psi data, for various ranges of transverse momentum, are compared to theoretical models incorporating colour-singlet and colour-octet matrix elements. Predictions of a next-to-leading-order colour-singlet model give a good description of the data, although there is a large normalisation uncertainty. The J/psi helicity distribution for z > 0.4 is compared to leading-order QCD predictions.

Figures (11)

• Figure 1: a) The direct photon process at leading order in the CS framework; b) the resolved photon process in the same framework; c) the direct photon process in the CO framework.
• Figure 2: The invariant-mass spectrum measured in the region 50 $< W <$ 180 GeV for a) 0.1 $< z <$ 0.55, b) 0.55 $< z <$ 0.9 and c) 0.9 $< z <$ 1. The signal regions are shown as the shaded bands and the background as the continuous line.
• Figure 3: Number of events reconstructed in the kinematic region $z >$ 0.4 and 50 $< W <$ 180 GeV plotted against a) $J/\psi$$p_T$, b) $W$ c) inelasticity, $z$ and d) $J/\psi$ polar angle, $\theta$. The data distributions are shown as the points with statistical errors only. The simulated EPSOFT diffractive proton-dissociation background is shown as the dotted lines. The solid lines show the prediction of the sum of the HERWIG and EPSOFT generators. The combined MC has been area normalised to the data. The HERWIG MC sample was reweighted in $p_T$ and $W$ to the data.
• Figure 4: $\psi^{\prime}$ to $J/\psi$ cross-section ratios as a function of a) $p_T$, b) $W$, and c) $z$, measured for $50 < W < 180$ GeV and $0.55 < z < 0.9$. The uncertainties are statistical only.
• Figure 5: Differential cross-sections $d\sigma/dz$ for $50 < W < 180$ GeV and different $p_T$ selections: no $p_T$ cut (squares), $p_T > 1$ (circles), and $p_T > 2$ GeV (triangles). The inner error bars show the statistical uncertainty; the outer bars show the statistical and systematic uncertainties added in quadrature.