Exclusive photoproduction 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 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${\rm pb}^{-1}$ 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 $\alpha_{\mathbb{P}}= 0.115 \pm 0.018(stat.)^{+0.008}_{-0.015}(syst.) {\rm GeV}^{-2}$.