Speaker
Description
On behalf of the ZEUS Collaboration.
Submitted to JHEP.
The exclusive photoproduction reactions gamm p -> J/psi(1S) p and gamma p -> psi(2S) p have been measured at an ep centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 373 pb-1. The measurement was made in the kinematic range 30 < W < 180 GeV, Q^2 < 1 GeV^2 and |t| < 1 GeV^2, where W is the photon–proton centre-of-mass energy, Q^2 is the photon virtuality and t is the squared four-momentum transfer at the proton vertex. The decay channels used were J/psi(1S) -> mu+mu−, psi(2S) -> mu+mu− and psi(2S) -> J/psi(1S)pi+pi− with subsequent decay J/psi(1S) -> mu+mu−. The ratio of the production cross sections, R = sigma(psi(2S))/sigma(J/psi(1S)), has been measured as a function of W and |t| and compared to previous data in photoproduction and deep inelastic scattering and with predictions of QCD-inspired models of exclusive vector-meson production, which are in reasonable agreement with the data.