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

The exclusive photoproduction reaction \gamma p \rightarrow \psi(2S) pγp→ψ(2S)phas been studied with the ZEUS detector in epep collisions at HERAusing an integrated luminosity of 333~pb^{-1} −1 , in the kinematic range30 < W < 18030<W<180~GeV, Q^2 < 1Q 2 <1~GeV^2 2 , |t| < 5∣t∣<5~GeV^2 2 , where WW is the photon proton centre-of-mass energy, Q^2Q 2 - the photon virtuality and tt - four-momentum transfer at the proton vertex.The \psi(2S)ψ(2S) mesons were identified via the decay channels:\psi(2S) \rightarrow \mu^+ \mu^-ψ(2S)→μ + μ − and J/\psi\, \pi^+ \pi^-J/ψπ + π − with J/\psi \rightarrow \mu^+ \mu^-J/ψ→μ + μ − .The ratio of the production cross sections R = \sigma(\psi(2S)) / \sigma(J/\psi)R=σ(ψ(2S))/σ(J/ψ)was measured as a function of WW, for three WW intervals: 30 < W < 8030<W<80~GeV, 80 < W <13080<W<130~GeV and 130 < W < 180130<W<180~GeV and in the whole kinematicrange accessible in this analysis.The advantage of including the 4-prong final state channel is its negligiblebackground in the respective invariant mass distribution and small systematicuncertainty due to the branching ratio (BR) comparing to the BR of directdecay of \psi(2S)ψ(2S) into a muon pair. The presented analysis is complementary toalready published ZEUS measurement of the same quantity as a function of Q^2Q 2 in deep-inelastic (DIS) channel.