Paper
Measurement of Elastic $J/ψ$ Photoproduction at HERA
Authors
M. Derrick, S. Magill, D. Mikunas, B. Musgrave, J. Repond, R. Stanek, R. Talaga, R. Yoshida, H. Zhang, G. Bruni, G. Romeo, G. Castellini, L. Cifarelli, F. Cindolo, S. Pasquale, I. Gialas, Federico Palmonari, A. Pesci, A. Polini, G. Sartorelli, Y. Garcia, A. Zichichi, L. Feld, M. Pfeiffer, C. Rembser, J. Stamm, R. Wedemeyer, S. Ritz, M. Löwe, J. Mainusch, T. Monteiro, J. Ng, K. Ohrenberg, I. Park, W. Schulz, T. Voss, U. Wollmer
Abstract
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*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 in the form sigma_{gamma p -> J/Psi p} \propto W^δ yields the value δ= 0.92 \pm 0.14 \pm 0.10. The differential cross section dsigma/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. dσ/d|t| falls exponentially with a slope parameter of 4.6 \pm 0.4 (+0.4-0.6) GeV^{-2}. The measured decay angular distributions are consistent with s-channel helicity conservation.