M. Derrick et al., MEASUREMENT OF THE CROSS-SECTION FOR THE REACTION GAMMA-P-]J PSI-P WITH THE ZEUS DETECTOR AT HERA/, Physics letters. Section B, 350(1), 1995, pp. 120-134
This paper reports the cross section measurements for the process ep -
-> e J/psi p for Q(2) < 4 GeV2 at root s = 296 GeV, based on an integr
ated luminosity of about 0.5 pb(-1), using the ZEUS detector. The J/ps
i was detected in its e(+)e(-) and mu(+)mu(-) decay modes. The photopr
oduction cross section was measured to be 52(-12)(+7) +/- 10 nb at an
average gamma p centre of mass energy of 67 GeV and 71(-20)(+13) +/- 1
2 nb at 114 GeV. The significant rise of the cross section compared to
lower energy measurements is not in agreement with VDM models, but ca
n be described by QCD inspired models if a rise in the gluon momentum
density at low x in the proton is assumed.