Exclusive electroproduction of J/PSI mesons at HERA

The exclusive photoproduction of \(J/\psi\) mesons, \(\gamma p \rightarrow J/\psi p\), has been studied in ep collisions with the ZEUS detector at HERA, in the kinematic range 20J/\psi\) mesons were reconstructed in the muon and the electron decay channels using integrated luminosities of 38\({\rm pb}^{-1}\) and 55\({\rm pb}^{-1}\),respectively. The helicity structure of \(J/\psi\) production shows that the hypothesis of s-channel helicity conservation is satisfied within two standard deviations. The total cross section and the differential cross-section \(d\sigma/dt\), where t is the squared four-momentum transfer at the proton vertex, are presented as a function of W, for |t|<1.8 GeV2. The t distribution exhibits an exponential shape with a slope parameter increasing logarithmically with W with a value \(b=4.15 \pm 0.05 (stat.)^{+0.30}_{-0.18} (syst.){\rm GeV}^{-2}\) at \(W=90{\rm GeV}\). The effective parameters of the pomeron trajectory are \(\alpha_{\mathhbb{P}}(0) = 1.200 \pm 0.009(stat.)^{+0.004}_{-0.010}(syst.)\) and \(\alpha_{\mathbb{P}}= 0.115 \pm 0.018(stat.)^{+0.008}_{-0.015}(syst.) {\rm GeV}^{-2}\).