Measurement of inelastic J/#psi# photoproduction at HERA

We present a measurement of the inelastic, non diffractive J/#psi# photoproduction cross section in the reaction e"+p#->#e"+J/#psi#X with the ZEUS detector at HERA. The J/#psi# was identified using both the #mu#"+#mu#"- and e"+e"- decay channels and events were selected within the range 0.4<z<0.9 (0.5<z<0.9) for the muon (electron) decay mode, where z is the fraction of the photon energy carried by the J/#psi# in the proton rest frame. The cross section, the p"2_T and the z distributions, after having subtracted the contributions from resolved photon and diffractive proton dissociative processes, are given for the photon-proton centre of mass energy range 50<W<180 GeV; p"2_T is the square of the J/#psi# transverse momentum with respect to the incoming proton beam direction. In the kinematic range 0.4<z<0.9 and p"2_T>1GeV"2, NLO calculations of the photon-gluon fusion process based on the colour-singlet model are in good agreement with the data. The predictions of a specific leading order colour-octet model, as formulated to describe the CDF data on J/#psi# hadroproduction, are not consistent with the data. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman