Exclusive photoproduction of J/#psi# mesons at HERA

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 d#sigma#/dt, where t is the squared four-momentum transfer at the proton vertex, are presented as a function of W, for vertical stroke t vertical stroke < 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 #+-# 0.05(stat.)"+"0"."3"0_-_0_._1_8(syst.) GeV"-"2 at W = 90 GeV. The effective parameters of the Pomeron trajectory are #alpha#_I_P (0) 1.200#+-#0.009(stat.)"+"0"."0"0"4_-_0_._0_1_0(syst.) and #alpha#'_I_P 0.115#+-#0.018(stat.)"+"0"."0"0"8_-_0_._0_1_5(syst.) GeV"-"2. (orig.)Available from TIB Hannover: RA 2999(02-008) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman