Exclusive electroproduction of J/psi mesons at HERA

TL;DR

This study measures the cross sections and angular distributions of exclusive J/psi meson electroproduction at HERA, providing insights into the gluon distribution in the proton and testing QCD predictions across various photon virtualities.

Contribution

It presents new measurements of cross sections and angular distributions over a wide Q^2 range, confirming the applicability of perturbative QCD and the sensitivity to gluon distributions.

Findings

Effective Pomeron parameters agree with photoproduction results.

Spin-density matrix elements support s-channel helicity conservation.

The ratio sigma_L/sigma_T increases with Q^2.

Abstract

The exclusive electroproduction of J/psi mesons, ep->epJ/psi, has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges 0.15<Q^2<0.8 GeV^2 and 2<Q^2<100 GeV^2 using integrated luminosities of 69 pb^-1 and 83 pb^-1, respectively.The photon-proton centre-of-mass energy was in the range 30<W<220 GeV and the squared four-momentum transfer at the proton vertex |t|<1.The cross sections and decay angular distributions are presented as functions of Q^2, W and t. The effective parameters of the Pomeron trajectory are in agreement with those found in J/psi photoproduction. The spin-density matrix elements, calculated from the decay angular distributions, are consistent with the hypothesis of s-channel helicity conservation. The ratio of the longitudinal to transverse cross sections, sigma_L/sigma_T, grows with Q^2, whilst no dependence on W or t is observed. The results are in agreement with perturbative QCD calculations and exhibit a strong sensitivity to the gluon distribution in the proton.