Measurement of elastic J/#psi# photoproduction at HERA

The reaction #gamma# p #-># J/#psi#p has been studied in ep interactions 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 photon virtuality Q"2 of 5 x 10"-"5 GeV"2. The photoproduction cross section, #sigma#_#gamma#_p_#->#_J_/_#psi#_p, is observed to rise steeply with W. A fit to the data presented in this paper to determine the parameter #delta# in the form #sigma#_#gamma#_p_#->#_J_/_#psi#_p #propor to# W"#delta# yields the value #delta# = 0.92 #+-# 0.14 #+-# 0.10. The differential cross section d#sigma#/d vertical stroke t vertical stroke is presented over the range vertical stroke t vertical stroke < 1.0 GeV"2 where t is the square of the four-momentum exchanged at the proton vertex. d#sigma#/d vertical stroke t vertical stroke falls exponentially with a slope parameter of 4.6 #+-# 0.4"+"0"."4_-_0_._6 GeV"-"2. The measured decay angular distributions are consistent with s-channel helicity conservation. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman