2020 Impact factor 4.590
Particles and Fields

Eur. Phys. J. C 24, 345-360 (2002)
DOI: 10.1007/s10052-002-0953-7

Exclusive photoproduction of $J/\psi$ mesons at HERA

The ZEUS collaboration

(Received: 1 February 2002 / Published online: 22 May 2002)

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 $20<\mbox{${W}$ }<290$ GeV, where $\mbox{${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 $\,\text{pb}^{-1}$ and 55 $\,\text{pb}^{-1}$, 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-sec tion $d\sigma/dt$, where t is the squared four-momentum transfer at the proton vertex, are presented as a function of W, for |t|<1.8 GeV 2. The t distribution exhibits an exponential shape with a slope parameter increasing logarithmically with W with a value $b=4.15 \pm 0.05 (stat.)^{+0.30}_{-0.18} (syst.){\,\text{Ge}\text{V}}^{-2}$ at $W=90{\,\text{Ge}\text{V}}$. The effective parameters of the Pomeron trajectory are $\alpha_{P}(0) = 1.200 \pm 0.009(stat.)^{+0.004}_{-0.010}(syst.)$ and $\alpha^\prime_{P} = 0.115 \pm 0.018(stat.)^{+0.008}_{-0.015}(syst.)~{\rm GeV}^{-2}$.

© Società Italiana di Fisica, Springer-Verlag 2002