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 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 GeV2 and |t| < 1 GeV2, 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.