Inelastic Photoproduction of J/Psi Mesons at HERA

TL;DR

This paper presents a detailed analysis of inelastic J/Psi meson photoproduction at HERA, measuring cross sections across a wide kinematic range and comparing results with various theoretical models, including Colour Singlet and NRQCD frameworks.

Contribution

It provides the first measurements of cross sections at z<0.3 and tests multiple theoretical models against extensive experimental data.

Findings

Good agreement between data and NLO Colour Singlet Model in medium z range

LO k_t factorisation approach describes the data well

Data and theory are reconcilable with modest color octet contributions

Abstract

An analysis of inelastic photoproduction of J/Psi mesons is presented using data collected at the ep collider HERA corresponding to an integrated luminosity of above 80pb-1. Differential and double differential cross sections are measured in a wide kinematic region: 60<W_gammap<260 GeV, 1<p_t^2< 60 GeV^2 and 0.05<z<0.9, where z is the fraction of the energy of the exchanged photon transferred to the J/Psi meson in the rest frame of the target proton. Cross sections at z<0.3 are presented for the first time. Theoretical calculations within the Colour Singlet Model at NLO for direct photon processes are shown to give a good description of the data in the medium z region (0.3<z<0.9) up to the highest p_t^2 values. A calculation using a k_t factorisation approach in LO in the Colour Singlet Model is also able to describe these data. The data in the full z range are also compared to LO calculations within a non-relativistic QCD framework including color octet and colour singlet contributions for direct and resolved photons. It seems possible to reconcile data and theory with modest contributions from colour octet processes. The polarisation of the J/Psi meson is measured as a function of z and p_t,psi and is reasonably described by the theoretical predictions.