Measurement of the cross-section ratio σ_ψ(2S) /σ_J/ψ(1S) in exclusive photoproduction at HERA

The exclusive photoproduction reactions γp → J/ψ(1S)p and γp → ψ(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/ψ(1S) → μ⁺μ⁻, ψ(2S) → μ⁺μ⁻ and ψ(2S) → J/ψ(1S)π⁺π⁻ with subsequent decay J/ψ(1S) → μ⁺μ⁻. The ratio of the production cross sections, R = σ_ψ(2S)/σ_J/ψ(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.