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

TL;DR

This paper measures the ratio of cross sections for exclusive photoproduction of $cpsi(2S)e$ and $J/cpsi(1S)e$ at HERA, providing insights into vector meson production and testing QCD models.

Contribution

It provides the first detailed measurement of the cross-section ratio as a function of energy and momentum transfer, comparing results with theoretical predictions.

Findings

The ratio $R$ varies with $W$ and $|t|$ in a manner consistent with QCD models.

Results agree reasonably with previous photoproduction and deep inelastic scattering data.

The data support current theoretical understanding of exclusive vector meson production.

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, $Q^2 < 1$ 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.