Measurement of the cross-section ratio $σ_{ψ(2S )}/σ_{J/ψ(1S )}$ in exclusive photoproduction at HERA (2206.13343v2)
Abstract: 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, $Q2 < 1$ GeV$2$ and $|t| < 1$ GeV$2$, 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 $J/\psi(1S) \to \mu+ \mu-$, $\psi(2S) \to \mu+ \mu-$ 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.