Exclusive photoproduction of J/psi mesons at HERA

TL;DR

This paper reports on the measurement of exclusive J/psi meson photoproduction at HERA, analyzing cross sections, t-distributions, and Pomeron trajectory parameters to understand the underlying QCD processes.

Contribution

It provides detailed measurements of cross sections and t-distributions for J/psi production, including the Pomeron trajectory parameters, in a previously unexplored kinematic range.

Findings

Helicity conservation hypothesis is satisfied at two standard deviations.

The t distribution follows an exponential shape with a W-dependent slope.

Pomeron trajectory parameters are precisely measured.

Abstract

The exclusive photoproduction of J/psi mesons, gamma p->J/psi p, has been studied in ep collisions with the ZEUS detector at HERA, in the kinematic range 20<W<290 GeV, where W is the photon-proton centre-of-mass energy. The J/psi mesons were reconstructed in the muon and the electron decay channels using integrated luminosities of 38 pb^-1 and 55 pb^-1, respectively. The helicity structure of J/psi production shows that the hypothesis of s-channel helicity conservation is satisfied at the two standard-deviation level. The total cross section and the differential cross-section dsigma/dt, where t is the squared four-momentum transfer at the proton vertex, are presented as a function of W, for |t|<1.8 GeV^2. The t distribution exhibits an exponential shape with a slope parameter increasing logarithmically with W with a value b=4.15 \pm 0.05 (stat.)^{+0.30}_{-0.18} (syst.) GeV^-2 at W=90 GeV. The effective parameters of the Pomeron trajectory are alphapom(0) = 1.200 \pm 0.009(stat.)^{+0.004}_{-0.010}(syst.) and alphappom= 0.115 \pm 0.018(stat.)^{+0.008}_{-0.015}(syst.) GeV^-2.