Exclusive Electroproduction of J/ψ Mesons at-HERA

The exclusive electroproduction of J/ψ mesons, ep→epJ/ψ, has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges 0.15<Q2<0.8 GeV2 and 2<Q2<100 GeV2 using integrated luminosities of 69 and 83 pb-1, respectively. The photon-proton centre-of-mass energy was in the range 30<W<220 GeV and the squared four-momentum transfer at the proton vertex t <1 GeV2. The cross sections and decay angular distributions are presented as functions of Q2, W and t. The effective parameters of the pomeron trajectory are in agreement with those found in J/ψ photoproduction. The spin-density matrix elements, calculated from the decay angular distributions, are consistent with the hypothesis of s-channel helicity conservation. The ratio of the longitudinal to transverse cross sections, σL/σT, grows with Q2, whilst no dependence on W or t is observed. The results are in agreement with perturbative QCD calculations and exhibit a strong sensitivity to the gluon distribution in the proton.