Measurement of the $\psi(2S)$ to $J/\psi$ cross section ratio in photoproduction with the ZEUS detector at HERA

Abstract

The exclusive photoproduction reaction $\gamma p \rightarrow \psi(2S)p$ has been studied with the ZEUS detector in ep collisions at HERA using an integrated luminosity of 333 pb$^{−1}$, in the kinematic range 30 < W < 180 GeV, Q$^2$ < 1 GeV$^2$ , |t| < 5 GeV$^2$ , where W is the photon proton centre-of-mass energy, Q$^2$ - the photon virtuality and t - four-momentum transfer at the proton vertex. The $\psi(2S)$ mesons were identified via the decay channels: $\psi(2S)\rightarrow \mu^+ \mu^-$ and $J/\psi \pi^+\pi^-$ with $J/\psi \rightarrow \mu^+\mu^-$. The ratio of the production cross sections $R = \sigma(\psi(2S))/\sigma(J/\psi)$ was measured as a function of W, for three W intervals: 30 < W < 80 GeV, 80 < W < 130 GeV and 130 < W < 180 GeV and in the whole kinematic range accessible in this analysis. The advantage of including the 4-prong final state channel is its negligible background in the respective invariant mass distribution and small systematic uncertainty due to the branching ratio (BR) comparing to the BR of direct decay of $\psi(2S)$ into a muon pair. The presented analysis is complementary to already published ZEUS measurement of the same quantity as a function of $Q^2$ in deep-inelastic (DIS) channel [1].