Measurement of the cross-section ratio $\ddot{I}{ƒ}_{\ddot{I}ˆ(2S)}/\ddot{I}{ƒ}_{J/\ddot{I}ˆ(1S)}$ in exclusive photoproduction at HERA

Abstract

The exclusive photoproduction reactions ${\hat{I}}^{3}p\to J/\ddot{I}ˆ(1S)p$ and ${\hat{I}}^{3}p\to \ddot{I}ˆ(2S)p$ 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 $J/\ddot{I}ˆ(1S)\to \hat{I}{¼}^{+}\hat{I}{¼}^{-}$, $\ddot{I}ˆ(2S)\to \hat{I}{¼}^{+}\hat{I}{¼}^{-}$ and $\ddot{I}ˆ(2S)\to J/\ddot{I}ˆ(1S)\ddot{I}{€}^{+}\ddot{I}{€}^{-}$ with subsequent decay $J/\ddot{I}ˆ(1S)\to \hat{I}{¼}^{+}\hat{I}{¼}^{-}$. The ratio of the production cross sections, $R=\ddot{I}{ƒ}_{\ddot{I}ˆ(2S)}/\ddot{I}{ƒ}_{J/\ddot{I}ˆ(1S)}$, 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.

31 pages, 9 figures. Updated according to comments from journal referee including a new table. Added full collaboration author list