Speaker
Description
On behalf of the ZEUS Collaboration.
The exclusive photoproduction reactions γp → J/ψ(1S)p and γp → ψ(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, Q2 < 1 GeV2 and |t| < 1 GeV2, where
W is the photon–proton centre-of-mass energy, Q2 is the photon virtuality and t
is the squared four-momentum transfer at the proton vertex. The decay channels
used were J/ψ(1S) → μ+μ−, ψ(2S) → μ+μ− and ψ(2S) → J/ψ(1S)π+π− with
subsequent decay J/ψ(1S) → μ+μ−. The ratio of the production cross sections,
R = σψ(2S)/σJ/ψ(1S), has been measured as a function of W and |t| and compared
to previous data in photoproduction and deep inelastic scattering and with predic-
tions of QCD-inspired models of exclusive vector-meson production, which are in
reasonable agreement with the data
