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

ZEUS Collaboration
Jun 2022
摘要
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 energyof 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 373pb$^{-1}$. 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--protoncentre-of-mass energy, $Q^2$ is the photon virtuality and $t$ is the squaredfour-momentum transfer at the proton vertex. The decay channels used were$J/\psi(1S) \to \mu^+ \mu^-$, $\psi(2S) \to \mu^+ \mu^-$ and $\psi(2S) \toJ/\psi(1S) \pi^+ \pi^-$ with subsequent decay $J/\psi(1S) \to \mu^+ \mu^-$. Theratio of the production cross sections, $R = \sigma_{\psi(2S)} /\sigma_{J/\psi(1S)}$, has been measured as a function of $W$ and $|t|$ andcompared to previous data in photoproduction and deep inelastic scattering andwith predictions of QCD-inspired models of exclusive vector-meson production,which are in reasonable agreement with the data.
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