Measurement of the cross-section ratio \(\sigma_{\psi(2S )}/\sigma_{J/\psi(1S )}\) in exclusive photoproduction at HERA

The exclusive photoproduction reactions \(\gamma p \to J/\psi(1S) p\) and \(\gamma p \to \psi(2S) p\) 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 \(J/\psi(1S) \to \mu^+ \mu^-\), \(\psi(2S) \to \mu^+ \mu^-\) and \(\psi(2S) \to J/\psi(1S) \pi^+ \pi^-\) with subsequent decay \(J/\psi(1S) \to \mu^+ \mu^-\). 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.