Measurement of the cross-section ratio sigma(Psi(2S))/sigma(J/Psi(1S)) in exclusive photoproduction at HERA

The exclusive photoproduction reactions gamma p --> J/ Psi(1S)p and gamma + 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, Q2 < 1 GeV2 and |t| < 1 GeV2, where W is the photon-proton centre-of-mass energy, Q2 is the photon virtuality and t is the squared four momentum transfer at the proton vertex. The decay channels used were J/Psi(1S) --> μ+μ−, Psi(2S) --> μ+μ− and Psi(2S) --> J/Psi(1S) pi+ pi− with subsequent decay J/Psi(1S) --> μ+μ−. 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.