DIFFRACTIVE J PSI PHOTOPRODUCTION AS A PROBE OF THE GLUON DENSITY/

We use perturbative QCD, beyond the leading In Q(2) approximation, to show how measurements of diffractive J/psi production at HERA can prov ide a sensitive probe of the gluon density of the proton at small valu es of Bjorken x. We estimate both the effect of the relativistic motio n of the c and (c) over bar within the J/psi and of the rescattering o f the <c(c)over bar> quark pair on the proton. We find that the availa ble data for diffractive J/psi photoproduction can discriminate betwee n the gluon distributions of the most recent sets of partons.