J. Breitweg et al., MEASUREMENT OF ELASTIC J UPSILON PHOTOPRODUCTION AT HERA/, Zeitschrift fur Physik. C, Particles and fields, 75(2), 1997, pp. 215-228
The reaction a p --> J/psi p has been studied in epsilon p interaction
s using the ZEUS detector at HERA. The cross section for elastic J/psi
photoproduction has been measured as a function of the photon-proton
centre of mass energy W in the range 40 < W < 140 GeV at a median phot
on virtuality Q(2) of 5 x 10(-5) GEV(2). The photoproduction cross sec
tion, sigma(ap-->J/psi p), is observed to rise steeply with W. A fit t
o the data presented in this paper to determine the parameter delta in
the form sigma(ap-->J/psi p) proportional to W-delta yields the value
delta=0.92+/-0.14+/-0.10. The differential cross section d sigma/d\t\
is presented over the range \t\ < 1.0 GEV(2) where t is the square of
the four-momentum exchanged at the proton vertex. d sigma/d\t\ falls
exponentially with a slope parameter of 4.6+/-0.4(-0.6)(+0.4) GEV(-2).
The measured decay angular distrubitions are consistent with 8-channe
l helicity conservation.