J/psi production at HERA

TL;DR

This paper analyzes J/psi production at HERA, comparing experimental data with next-to-leading order predictions, revealing discrepancies in production rates and polarization predictions.

Contribution

It provides a detailed comparison of NLO predictions with experimental data, highlighting the limitations of the color-singlet model in describing J/psi production.

Findings

Experimental rates exceed color-singlet predictions.

NLO predicts mostly longitudinal polarization at high transverse momentum.

Shapes of distributions are well reproduced.

Abstract

We consider the J/psi photo-production data collected at HERA in the light of next-to-leading order predictions for the color-singlet yield and polarization. We find that, while the shapes of inclusive distributions in the transverse momentum and inelasticity are well reproduced, the experimental rates are larger than those given by the color-singlet contribution alone. Furthermore, the next-to-leading order calculation predicts the J/psi's to be mostly longitudinally polarized at high transverse momentum in contrast with the trend of the preliminary data from the ZEUS collaboration.