Exclusive electroproduction of J/ ψ mesons at HERA

Abstract

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<Q 2<0.8 GeV 2 and 2<Q 2<100 GeV 2 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 GeV 2 . The cross sections and decay angular distributions are presented as functions of Q 2, 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, σ L / σ T , grows with Q 2, 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.